The symbol "$:=$" seems to have been introduced in programming languages in the 1960's. For instance in Pascal, one writes $x = 0$ for testing equality (like in "if $x=0$ then...") and $x := 0$ to assign the value $0$ to the variable $x$.
However, since assignment is more frequent than equality testing, languages like C or Java use a different syntax: $x = 0$ for assignment and $x == 0$ for equality testing.
After that, the notation $:=$ spread out in mathematical writing, mostly to mean "equal by definition". I would not recommend using it, but it is nevertheless quite common.
EDIT. According to the Wikipedia entry ALGOL 58, ALGOL 58, originally known as IAL, is one of the family of ALGOL computer programming languages.
The distinction between assignment (:= representing a left-facing
arrow) and the equality relation (=) was introduced in IAL and kept in
Thus the use of := in computer science goes back to at least 1958.