<em>Until</em> is the full standard word: <em>The offer runs until Friday.</em> <em>Til</em> is a nonstandard spelling of the casual clipping, which is properly written <em>'til</em>: <em>Two days 'til the results.</em> They mean exactly the same thing; only the register and the correctness differ.
The core difference
There is no difference in meaning at all. Both point to the moment when a state or action stops: you wait, work, sleep or hold on up to a given time. The difference is entirely one of register and spelling status.
Until is accepted everywhere. Til, with no apostrophe, is not listed as standard and reads as a typo to most editors.
How to use each
- until — anything you would show an employer, a lecturer or a client: Payment is not due until the thirtieth. / She didn't speak until the room was empty.
- 'til — deliberate informality: captions, lyrics, texts, chalkboards. Open 'til late.
- til — best avoided; use one of the two above.
A test for which register you're in
Ask whether the sentence could carry an exclamation mark without embarrassment. Only one sleep 'til Christmas! works. Interest accrues 'til maturity! does not — that sentence wants until.
If you want a form that is short and always safe, use till. It is a standard word with the same meaning, it needs no apostrophe, and it is a letter shorter than until.
Where until earns its extra syllable
Two places genuinely favour the full form. At the start of a sentence, Until reads more cleanly than the alternatives: Until the audit is finished, no invoices go out. And in negative constructions where the timing carries weight, the full word gives the sentence more spine: He did not admit it until confronted.
The common mistake
The real trap is doubling up. Up until is very common in speech and is not wrong, but it is padded — until already contains the sense of "up to". Trim it unless you are drawing a deliberate contrast with what happened afterwards.
The other slip is using until for distance rather than time. English marks distance with up to or as far as: We walked as far as the bridge, not "until the bridge". Time and sequence take until; space usually does not.
Frequently Asked Questions
Is "til" just a shorter way of writing "until"?
It is meant to be, but the accepted spelling of that clipping is 'til with an apostrophe. Bare til is treated as an error in edited writing, so use until, till, or 'til instead.
Is "up until" wrong?
Not wrong, just redundant in most sentences, since until already means up to. Keep it only when you want to stress the whole stretch of time before a change: "up until last year, we did it by hand".
Which should I use in a formal email — until or 'til?
Until. The clipped form signals casualness, which is fine in a text to a friend but out of place in professional writing.