Pseudorandom permutation (PRP)
A permutation that an adversary with limited access cannot tell apart from a random permutation.
A keyed block cipher is an attempt to build a pseudorandom permutation (PRP). The ideal would be a random permutation of all blocks, but a table of that kind for n-bit blocks has 2n entries and cannot be stored. Luby and Rackoff showed in 1988 that three Feistel rounds built from pseudorandom functions give a PRP, and that four rounds give a stronger variant.
The difference from a PRF is that a permutation is invertible, which a block cipher needs to be. The notion is the standard security model when proving that modes such as CBC are secure, provided the block cipher behaves like a PRP.