When searchers type a query into a search engine, it isn’t uncommon for them to use more than one word. It also isn’t unusual for those words to be a semantically meaningful phrase rather than just a list of keywords.
Multiple search terms entered by a user are often more useful if considered by the search engine as a single compound unit. Assume that a user enters the search terms “baldur’s gate download.”
The user intends for this query to return web pages that are relevant to the user’s intention of downloading the computer game called “baldur’s gate.” Although “baldur’s gate” includes two words, the two words together form a single semantically meaningful unit.
If the search engine is able to recognize “baldur’s gate” as a single semantic unit, called a compound herein, the search engine is more likely to return the web pages desired by the user.