In a dictionary or similar reference book, the headword is the word that is being defined. It is the equivalent of the title of the entry. Most dictionaries print the headword in bold type, and with a lower case first letter.
The OED and other scholarly works use the word lemma to name the same part of their dictionaries. OED gives the following meaning: "a definiendum", which is a Latin formation meaning 'that which is to be defined'. (Lemma has other, technical, meanings in Maths and Biology.)