The crossword solution for the term 'ending for HERO or SERPENT' is INE, which is a three-letter word. INE is a common suffix used to form adjectives and nouns from other words. It is derived from Latin and is often used to denote a quality or characteristic of something. For example, the word 'HEROINE' is formed by adding the suffix INE to the word 'HERO'. Similarly, the word 'SERPENTINE' is formed by adding the suffix INE to the word 'SERPENT'.