A member of the House of Commons who is not a party leader WordNet 3.0 & Open English WordNet

In Westminster and other parliamentary systems, a backbencher is a member of parliament or a legislator who occupies no governmental office and is not a frontbench spokesperson in the Opposition, being instead simply a member of the "rank and file". Wikipedia

An elected politician in a parliamentary system who is not a cabinet member Wikipedia Disambiguation

Parliamentarian who neither holds ministerial office nor shadows a minister Wikidata

A Member of Parliament who does not have cabinet rank, and who therefore sits on one of the backbenches or in one of the back rows of the legislature. Wiktionary

Member of Parliament who does not have cabinet rank. Wiktionary (translation)