Separated by a Common Language

Long Service Leave. I think this is thing in the UK, it is in Australia. Extra days of paid time off you get after you've been at an employer long enough.

Pandemic Leave - I've seen this used in two ways. First, paid time off when you are supposed to be isolating or quarantined after contact or a test. And also the more cynical - somebody who has been laid off because of lockdowns or loss of business.

Talking of familiarity breeding abbreviation, what does that say about the Australian sickie (Sick Leave)? Or for that matter, "chucking a sickie" - which is claiming to be sick when you're really doing something else.

US federal employees also say they're on annual leave, etc. Also can be marked AWOL. In my office we would joke about being on "in office annual", perhaps when assignments were lacking or one was not focusing on work as they should.

I know via friends that some US tech companies offer “sabbaticals” to their employees. As far as I know, these are not strictly for the employee to go off and do research, but I don’t know the boundaries of what these sabbaticals are allowed for. I have a feeling they might be akin to “study leave” in some cases.

Study leave can be the odd day here and there. A sabbatical, as I understand it, is a chunk

I'm British and have never been to the USA; I taught in universities in GB and Italy (I am now ancient), and everyone always said "sabbatical". I'd never heard of "research leave" until I read this post. H'm.

In frustration in a moment of crossness, I might say to my complaining partner to 'just leave it alone', meaning I am fed up with the discussion.

Just sticking in a comment-catching comment!

Compassionate leave. It may be more used in military contexts, but we would all understand it to mean permission to leave duty to attend a sick or dying close relative, for example.

I’ve only heard gardening leave in civil service (BrE) applications - a hiatus in which one’s specialist knowledge expires before starting a new position in business. or vice versa, I suppose.

The main use I know for "gardening leave" is when someone hands in their notice at work and is heading off to work for a competitor, so they are put on gardening leave so they can't contact clients to try to bring them to their new employer and so any insider information they have is out of date by the time they get there.

There's also (at least in the UK) "exam[ination] leave" for students to be taken off normal teaching schedules while they have exams.

In the same sense (permission) as in leave to remain, it can be necessary to seek leave to appeal certain types of court judgement.

Is 'unfirearmed' (or 'firearmed', or 'to firearm') really a word?

What about the classic Brexit nonsense: "Leave means leave"

A couple of other military-specific types of leave include:
embarkation and disembarkation leave - short periods of leave prior to or on completion of an overseas posting;
re-engagement leave - an additional allocation of days for having extended one's contract of military service
and terminal leave - somewhat akin to Unknown's long service leave.
There can also be block-leave - when the whole of a unit or military establishment closes down for a period of time, usually at tradutional holiday times of the year, somewhat similar to a factory closing for similar reasons, meaning that everyone is forced to take their holiday/vacation at the same time. I think the other important distinction with military leave is that it really does require permission to be actively given. Failure get permission results in the aforementioned AWOL. This is in contrast with many civilian work environments where the worker has greater freedom to unilaterally say when they wish to go on holiday/vacation/leave (except if you work in Japan, where as I understand it, the obligation to consider one's co-workers is paramount).

Zouk Delors said "Is 'unfirearmed' (or 'firearmed', or 'to firearm') really a word?"
Since 'armed' and 'unarmed' are acceptable forms from the verb to arm, being more specigfic about the type of weapon - a firearm - seems entirely reasonable. But I'm guessing you were joking...

In the US "study leave" would just be "away for training". This can be for the day or for an extended period of 3-4 months. Often just "training" would be used. E.g. my niece is in Texas for training for the next few weeks. My dept secretary will away from the office next week for training. Or many faculty use their summer time for training (study, learning new skills).

Every time I enter the UK on my US passport it gets stamped with Leave to Enter, which always gives me a laugh.

Here's another one; way leave.
It means permission to run services, such as broadband cables or water pipes across a property. Generally money changes hands to arrange it then future owners of the land are also bound by it. To my BrE ears it sounds old fashioned, but it's still the correct term

In Canada, union contracts - and management benefits - often contain many types of leaves: Long Service leaves, as mentioned above, Special Leave for those types of things that mean you have to take short unplanned periods off of work, e.g. when something breaks and you have to wait for the repair person (I live in the far north and special leave was often called "Propane leave" for those periods of time when it got so cold that you had to stay home and keep your propane from gelling) and Manager's leave being an additional vacation like leave that managers might get. Leave is easier to negotiate than wage increases. I have to say what freaks me out though is that I understand that in some parts of the states, there is no legislated right to maternity and paternity leave.

I was recently told on another forum (talking about out-of-office emails) that "I am on leave until [date]" would not be understood by Americans - something I wasn't previously aware of.

Argh, let's try that link again. Sorry!

There's a phrase 'by your leave' that must have something to do with permission, or rather the lack of it - "he walked straight past me without a by your leave" is said indignantly in music-hall routines (BrE).
I don't think I would use the phrase, or hear it in normal speech - is it archaic?

I would expect "without so much as a 'by your leave'". See https://www.phrases.org.uk/meanings/without-so-much-as-a-by-your-leave.html

Another type of leave is 'retention leave'. I'd never heard of it, because I work on a contract, but a staff member where I work (here in South Australia) recently referred to having accumulated 'annual, retention and long service leave', and they were going to use up all the leave before doing more work. I don't know how widespread the term is, but a definition from an SA government website is: 'A skills and experience leave entitlement provided to eligible long term employees who have completed 15 years of effective service.'

Two comments.

First, on wayleave and easement, technically in English usage, an easement is a right by the owner of one piece of land to do something on another piece of land, usually to cross it. The right, sort of, belongs to the land not the owner. It's a right of property and is usually permanent. A wayleave is a permission to do something, e.g. run a power cable or a gas main, across someone else's land but it belongs to the person given the right not the owner of a specific piece of land. It's not a right of property. It also may be temporary and be paid for by an annual fee rather than a capital payment.

Second 'administrative leave' - in the UK that's usually called being 'suspended'. Because it's ill regarded and legally risky to sack someone on the spot without investigating the circumstances and giving them the right to explain themselves and because letting people who have a disciplinary hanging over them work normally might also be disruptive or imply the employer isn't as uneasy as he or she claims to be if the person is allowed to carry on working as usual, the possible rotten apple might be suspended during the process. As it hasn't been determined whether they have done what they are accused of doing, they will often be entitled to go on being paid.

"The leave in all of these expressions is not "I'm leaving! Bye-bye!", but that you have been given leave (permission) to go."

I suspect few if any Americans know this. I think the general assumption here is that it's called "leave" because you leave the premises.

Where I work (US), there's Professional Leave (aka Staff Development leave). Which is for training, or conferences, or such. Short term, one day or a few days. Same as study leave?

As alluded to above, the US Federal Government uses the term leave for both military and civilian employees. We have Annual Leave, Sick Leave, Leave Without Pay (LWOP), and yes, we can be AWOL. I only learned about five years ago that this is not the normal terminology in the US when I had to ask what “PTO” means.