Each year the Oxford Dictionary combs through the language as it was used that year to select a word of the year. The team of experts that make this choice contains some lexicographers, consultants, the dictionary’s marketing team and their editorial staff. Since the Oxford dictionaries have both a US and a UK version they will each pick a word of the year. Sometimes they will both select the same word and other times each will have their own choice. This year they have both selected “post-truth”, which is when objective facts are less important than pleas to emotions.
As this word has been around for decades, it proves that the word does not have be coined that year, just have a spike in public uses. This year the Brexit and US Presidential elections drove more and more people to use the word which is what put it over the time. A few choices from the past have been selfie from 2013, shared by both the UK and US dictionaries, and in 2005 sudoku for the UK and podcast for the US.
A number of other groups select a word of the year as well. For example last year’s word of the year for Merriam-Webster was –ism. They have yet to name their word for 2016, but to give a couple points of comparison in 2013 they selected science and 2005 integrity was selected. Also the American Dialect Society will select a word each year with last year’s being the singular they. Again, they haven’t released this year’s just yet. But in 2013 they picked because when introducing a noun or adjective and in 2005 they selected truthiness. This group has been selecting words all the way back to 1990 when they chose bushlips. I’ll leave it up to you to track down its meaning.