The term '___ TO MENTION...' can be completed with two different words, 'NOT' (3 letters) and 'REFER' (5 letters). The phrase 'not to mention' is used to introduce an additional fact or comment that supports a previous statement. For example, 'I'm tired, not to mention hungry.' The phrase 'REFER to' is used to indicate that something is being mentioned or discussed. For example, 'I was referring to the time we went to the beach.'