Here the form mouse is the lemma, also called the citation form. The form lemma citation form mouse would also be the lemma for the word mice; dictionaries don't have separate definitions for inflected forms like mice. Similarly sing is the lemma for sing, sang, sung. In many languages the infinitive form is used as the lemma for the verb, so Spanish dormir "to sleep" is the lemma for duermes "you sleep". The specific forms sung or carpets or sing or duermes are called wordforms.