User:IssaRice/Colon-equals allows directionality

For definitions, I prefer $:=$ over other notations like $\equiv$, ${\overset {\Delta }{=}}$, and ${\overset {\text{def}}{=}}$. One reason is that $:=$ has the counterpart $=:$ that allows one to place the definiendum on the right. This allows one to e.g. write some long expression (during a calculation), then quickly assign it to something temporarily.

Leary & Kristiansen's logic textbook uses $:\equiv$ in the meta-level language.