What is the "etymology" of the notation ":="?

Question

I've noticed that sometimes people use ":=" to set variables, like "With $f(x):=x^{2}$, we have $f(1) = 1$." This is also the variable definition operation in Mathematica. My question is, did Mathematica come first and then people started using ":=" or vice versa? And when was the first documented use of such notation?

Answer 1

Edsger Dijkstra, who probably thought more about notation than most of us ever will, and was old enough to maybe know for sure, attributes the mathematical community's use of it as a borrowing from ALGOL (see this link).

For what it's worth, Dijkstra preferred to use $:=$ for assignment (which is actually its use in ALGOL), and if I am reading that opinion right, did not see the need for a "definitional" equality symbol (probably because if you write proofs in his favored style, "definitional" equality is explicitly signaled in words between formulas).