One-way encryption or one-way hash function is designed in a manner that it is hard to reverse the process, that is, to find a string that hashes to a given value (hence the name one-way). A good hash function makes it hard to find two strings that would produce the same hash value.
One way function
A one-way function is any function that is easy to compute on every input, but hard to invert given the image of a random input.