A frequently used word. For example: a, at, for, is, of, on, the. Search engines have, in the past, ignored these words to save time/resources when indexing. Search engines have evolved greatly since the early days, and stop words sometimes are meaningful, so this isn’t something to worry much about for SEO purposes.
Recommended reading:
- New Google Approach to Indexing and Stopwords by Bill Slawski
- How Google Might Ignore Insignificant Terms in Queries by Bill Slawski