Till and Until are equivalent. They mean the same thing. Till is not slang. It is perfectly correct to use till. Till and Until have been In use for hundreds of years. The variant 'til is sometimes used. I waited 'til he came. You can consider 'til as until without the un. There is some argument over the correctness, and necessity of 'til when you have till and until! Til was the original spelling of till. Chaucer used til. (Old and Middle English) For some reason Americans seem to feel that till is rather informal. Apparently not even Americans know why this is. Both til and till have been in the English language and considered correct for a very long time. I would welcome comments from Americans. Here is a link to start the discussion: http://grammarist.com/usage/until-till-til/

Til is the slang more informal use of until. They mean the same thing Example: Wait until 4 Wait til 4 One you would use with friends or family (til) the other you would use with people you don't know as well (until)