The Greek word theatron forms the root for our word

Our word theater is derived from the Greek word theatron, which contains the stem of the verb theasthai 'to view as spectators.

Theatre

Explanation:

In English, many of the words that we use come from ancient Greek and Latin words. This legacy can usually be seen in the "roots" of words. A root is a portion of a word from which other words grow when we add prefixes and suffixes. In this example, the word "theatron" is a Greek word that forms the root of the English word "theatre."