A lemma is a word that stands at the head of a definition in a dictionary. All the head words in a dictionary are lemmas. Technically, it is "a base word...

headword urus. The words aurochs, urus, and wisent  have all been used synonymously in English. AHD4, headwords aurochs, urus, wisent. MWU, headwords...

used in dictionaries as the headwords. Other forms of a lexeme are usually listed later in the entry. So, all headwords in a dictionary are lexemes....

be "colour" instead of "color". More exactly, Oxford spelling is the headwords used in their dictionaries. In the full entries other variations are listed...

“ A practice of class Vietnamese Animated Film (Bài thực-tập của lớp hoạt-họa Việt-nam) ” —Film's headword...