Till is slang. They mean the same thing. Until is more proper.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
| Main Entry: 2till Function: conjunction : UNTIL |
MrPedanticHere's Merriam-Webster's definition of "till":
Main Entry: 2till
Function: conjunction
: UNTIL
It doesn't say "slang"...
MrP