Alternative facts is a term used by Kellyanne Conway, a political consultant serving as Counselor to the President. This term was coined 2 days after Trump took office and is used as an alternative to the word lies. Here is a fictional elementary/middle school lesson on fact and opinion:
A fact is a statement that can be proven to be true. If I am wearing a yellow shirt, then that is a fact because you can prove that this is true by looking at the shirt I have on. An opinion is something that cannot be proven true or false because it is something that you think in your head. To the contrary of facts is an alternative fact; something that can be proven false. An example is that global warming is not real.
This statement has been used when Sean Spicer lied about the crowd size of the Trump inaguration. This is an alternative to the word lie.