The whole point of one way hashes is to provide a function where it is impossible to determine the input given the output. While any given input will always have the exact same output every time, a one way hash does not guarantee that given all possible inputs there will be an equal number of outputs. Thus, the strings produced are not necessarily unique.

For point of illustration, consider f(x) = 1. This is a one way hash. While not a very useful one, it does illustrate that not all values produced are unique.

Point taken. I was however speaking from an application programmer POV, which in this context i think validates my original statement.

I will go on to mention i'm not a technical wizard when its comes to math and crypto, and so your more technical definition is well recieved.

This one is easy! Look, the result of applying Digest::MD5 has always 128 bits, no matter how large the original data was. And of course the number of different 128-bit strings in this world is way less than the number of different strings of any length! Hope, this will help you to understand. Notice, no wizardry involved :)))