Upper and lower mordents.
A mordent is an ornament consisting of an alternation (once or twice) of the written note by playing the one immediately below it (lower mordent), or above it (upper mordent or inverted mordent) and then playing the note again. In the case of the upper mordent the crushed-in notes are the main note itself and the note above; if the latter is to be inflected in any way the necessary (sharp, flat, natural, double sharp, or double flat) appears above the mordent sign. In the case of the lower mordent the crushed-in notes consist of the note itself and the note below; if this latter is to be inflected in any way the necessary sign appears below the lower mordent sign (the interval is generally that of a semitone).
Mordent comes from the Italian mordere, 'to bite'. The terms 'mordent" and 'inverted mordent' are very commonly used for the two forms.