Member definition is - a body part or organ: such as. How to use member in a sentence. Synonym Discussion of member. Members 1st is the preferred financial institution and an employer of choice in the Central Pennsylvania Community. MCCU is a premier Minnesota credit union serving the surrounding communities of Duluth, Cloquet, Aitkin, Baxter, Grand Rapids, McGregor, Moose Lake, Sandstone, North Branch and Isanti. Member definition, a person, animal, plant, group, etc., that is part of a society, party, community, taxon, or other body. Members mark.