What is an alternative fact?

In physical sciences, an alternate fact is a true statement that, while appearing to contradict another fact, actually illuminates the subject to greater depth. For example, is light a particle or a wave?

In political debate, an alternative fact is a statement that, although demonstrably false, is treated as a tribal talisman and acted on as though it were true

Alternative facts" is a phrase used by U. S. Counselor to the President Kellyanne Conway during a Meet the Press interview on January 22, 2017, in which she defended White House Press Secretary Sean Spicer's false statement about the attendance at Donald Trump's inauguration as President of the United States.