Final answer:
A word in boldface type being explained in a dictionary is known as an 'entry word' or 'headword', with its definition, usage, and other details following it.
Step-by-step explanation:
The word in boldface type that is being explained in a dictionary is called an entry word or headword. In any dictionary, the entry word is the term that is being defined or described in detail, and it is typically set in boldface to make it stand out for easy reference.
When you look up a word, you'll find the headword in bold type at the beginning of the dictionary entry, followed by its pronunciation, part of speech, and one or more definitions.
In addition to the definition, dictionaries often provide additional information such as example sentences, etymology, and notes on usage.
Utilizing this structure allows users to effectively expand their vocabulary and understand the proper context and meaning of specific words.