1.NounThe act of beta reducing, an instance of replacing a function call by the result of calling a function.
2.NounOne of the three rewrite rules of the lambda calculus, which states that the application of a lambda abstraction
(
λ
x
.
t
)
{\displaystyle (\lambda x.t)}
to a term
s
{\displaystyle s}
yields an expression
t
[
s
/
x
]
{\displaystyle t[s/x]}
, "t with s instead of x", where all free instances of x in t have been replaced by s.