(covers information from several alternate timelines)
"It wasn't a smile. It was a smirk."
"Oh no, it was a smile. I could check the internal sensors if you'd like."
A smile was a facial expression that involved the turning up of one's lips, sometimes showing teeth. It could be a sign of happiness, stupidity, or intoxication. The act of forming or expressing a smile was called smiling.
The snowman created by Travis Mayweather was given a final touch by Malcolm Reed when he used a plasma torch to create a smile and eyes. (ENT: "Breaking the Ice") Ensign Travis Mayweather was smiling after he told the senior staff about life aboard a freighter and how his parents wound up with him. (ENT: "Fortunate Son")
In some instances, smiling was considered inappropriate. Following the announcement that President Abraham Lincoln was coming aboard the USS Enterprise, Captain Kirk threatened to "have the hide of the first man that so much as smiles" (at the absurdity of the situation). (TOS: "The Savage Curtain")