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.
- New Google Approach to Indexing and Stopwords by Bill Slawski
- How Google Might Ignore Insignificant Terms in Queries by Bill Slawski