A remix made by taking two different songs, usually by 2 seperate artists, and combining them into one.

a mash-up is slang for a get together of similarly minded people so that they might partake in the consumption of drugs and/or alcohol. people on a mash-up get mashed.
mash-ups usually take place at night, but usually go on well in to the morning of the following day (they tend to wind down when the alcohol/drugs run out). unlike a cotch, mash-ups can take place anywhere, in or outside.