Final answer:
In an alphabetic index, the term that follows the main term is known as a subterm. This subterm is used to provide more specific information or categorization related to the main term.
Step-by-step explanation:
When looking at an alphabetic index, especially in healthcare settings like coding manuals, the term that appears immediately following the main term or under the main term is typically the subterm. This subterm provides a more specific categorization or details related to the main term and is used to refine searches within the index, allowing for more precise indexing and cross-referencing. If we consider the provided options for the question 'What term appears in the alphabetic index immediately following the main term where it first appears either under the main term or subterm?', the answer would be C) Subterm.