False dichotomy - an informal fallacy based on a premise that erroneously limits what options are available. The source of the fallacy lies not in an invalid form of inference but in a false premise.

This definition I took from google is harder to understand than the word itself, so let's break it down in an example.

Let's say you are applying to a university. You think that you have only two options - uni X and uni Y, while in reality you have 420(supposedly) more universities to apply. Your preconception of having only 2 possible ways out is an example of a false dichotomy. 

Now, let's see how the word was formed to get it stamped on your memory. 

The word originates from Greek. Its initial form was "dikhotomia": 
"dikho" - "in two, apart", 
and "tomia" - "cutting". 

So if we combine these two translations, we get "cut in two". 

Finally, let's see how the phrase is used in real life.

1. Others warned of setting up a false dichotomy between copyright and freedom of expression.

2. If anything, his paintings abrade the false dichotomy between innocence and understanding.

Hopefully, that helps. Have a productive night(or day).