Theatre and theater are both nouns that mean a building, room, or outdoor structure for the presentation of plays, films, or other dramatic performances.
They can also refer to the activity or profession of acting in, producing, directing, or writing plays.
So, which one do you use and when?
For the most part, there is no demonstrable difference in sense or function between them. There are, however, important dialectal differences to consider, which are outlined below.
In a striking example of two cultures being divided by a common language, the UK and the US have each applied their own labyrinthine of rules, usages, and spellings to English. Thus, British English writers spell the word "theatre" while American English writers spell the word "theater".
So which spelling do you prefer?
Courtesy of: Writing Explained