1.NounA function for which every element of the codomain is mapped to by some element of the domain; (formally) Any function
f
:
X
→
Y
{\displaystyle f:X\rightarrow Y}
for which for every
y
∈
Y
{\displaystyle y\in Y}
, there is at least one
x
∈
X
{\displaystyle x\in X}
such that
f
(
x
)
=
y
{\displaystyle f(x)=y}
.