# Why do some use "$\;\stackrel{\text{def}}{=}\;$" for definitions? [duplicate]

Why do some use the equality symbol "$$\;\stackrel{\text{def}}{=}\;$$" for some definitions? For instance, $$\varepsilon \;\stackrel{\text{def}}{=}\; \frac{\Delta(L)}{L}$$

• In order to make a distinction with usage of "=" for properties ; example a formula like $S=\frac12$ height $\times$ base does not define area $S$ which has been defined elsewhere. Jun 6 '20 at 8:54
• You will also see definitions given by $:=$ symbol, which finds its origin in computer science languages affectation (even if affectation cannot be reduced to definition), more explicitly, Algol langage (en.wikipedia.org/wiki/Assignment_(computer_science)) Jun 6 '20 at 9:02
• You can also see "$\;\stackrel{\Delta}{=}\;$" for definitions. I prefer "$\;:=\;$", though. Much easier to type. :)
– Blue
Jun 6 '20 at 9:06
• @Blue indeed "$:=$" is better because it make clear what is defined by what, the other symbols are symmetric. By example, I used sometimes the symbol "$=:$" to define the meaning of a symbol after some algebra in the LHS of $=:$ Jun 6 '20 at 9:25
• A previous similar question here Jun 6 '20 at 9:43