Meaning of EXTENSION in English
n.1 the act or an instance of extending; the process of being extended.
2 prolongation; enlargement.
3 a part enlarging or added on to a main structure or building.
4 an additional part of anything.
5 a a subsidiary telephone on the same line as the main one. b its number.
6 a an additional period of time, esp. extending allowance for a project etc. b permission for the sale of alcoholic drinks until later than usual, granted to licensed premises on special occasions.
7 extramural instruction by a university or college (extension course).
8 extent, range.
9 Logic a group of things denoted by a term.
Etymology: ME f. LL extensio (as EXTEND)
Oxford English vocab. Оксфордский английский словарь. 2012