Till is not a shortened form of until. It is a complete word in its own right, and it is actually the older form in English. Until developed later.
That means both words are standard and correct. You can say Wait till noon or Wait until noon. Both are grammatical, and both mean the same thing in most contexts.
The difference is usually one of tone:
- Till often sounds a little more informal or conversational.
- Until can sound slightly more neutral or formal.
One common source of confusion is the spelling ’til. Many people use it as if till were a shortened form of until, but historically that is not how the words developed. If you want the standard full word, till is the better choice.
Examples:
- I will be here till Friday.
- I will be here until Friday.
- Do not open it till I call.
In short, till is fully correct, fully standard, and older than until.