An easily computed function whose inverse is extremely difficult to compute. An example is the multiplication of two large Primes. Finding and verifying two large Primes is easy, as is their multiplication. But factorization of the resultant product is very difficult.
See also RSA Encryption
Gardner, M. Chs. 13-14 in Penrose Tiles and Trapdoor Ciphers...and the Return of Dr. Matrix, reissue ed. New York: W. H. Freeman, pp. 299-300, 1989.