# What is many:1 and collision in MD5?

This is in reference to this answer on Stackoverflow. If you read the first paragraph of the answer, you'll find this line:

It's always a many:1 mapping (meaning there will always be collisions) since every function produces a smaller output than it's capable of inputting (If you feed every possible 1mb file into MD5, you'll get a ton of collisions).

What is this many:1 and what are collisions? Does many:1 mean two different inputs would have the same hash outputs?

Yes. This is a consequence of the pidgeonhole principle: as MD5 has a much larger number of potential inputs ($2^{2^{64}}$ or so) than outputs ($2^{128}$), some inputs must lead to the same output, i.e. collide.