What kind of encryption function is also used for decryption?
poncho and SEJPM more or less answered your question in the comments: If $f(f(m)) = m$, then the function can be revered to as a "self-inverse", or more formally, the function is said to be an "involution".

Be careful with a cipher that is an involution: There is a notable difference between a cipher that is a full involution and one that is not: Namely, if the cipher is an involution, then access to an encryption oracle is equivalent to access to a decryption oracle. 

A stream cipher with exclusive-or as the key stream combining function is a notable example of an involution: 

$c := E(m) = keystream \oplus m\\m := D(c) = E(c) = keystream \oplus c = keystream \oplus keystream \oplus m$