Just what you'd think. Another name or alias for a tag. For example, the tag Windows 2000 has the synonym W2K. When tagging content, if you enter a synonym of a tag (rather than the "main" name) you'll see that it gets replaced with the "main" name.
We aim to have the "main" name the one that is most commonly used and most descriptive. This may make things easier for the reader, who sees a clearer tag, and for the writer who may enter a shorter synonym for reasons of less keystrokes or because they're more familiar with a specific synonym. We hope this reduces and even eliminates content being tagged with one conceptual tag but exposed with all variants of its name.
Our search engine also takes synonyms into account. So when you search for W2K you'll get content tagged with Windows 2000.