转自：http://searchsoa.techtarget.com/definition/stop-word
In computer search engines, a stop word is a commonly used word (such as "the") that a search engine has been programmed to ignore, both when indexing entries for searching and when retrieving them as the result of a search query. When building the index, most engines are programmed to remove certain words from any index entry. The list of words that are not to be added is called a stop list. Stop words are deemed irrelevant for searching purposes because they occur frequently in the language for which the indexing engine has been tuned. In order to save both space and time, these words are dropped at indexing time and then ignored at search time. Some search engines allow you to include a stop word in your search by putting an inclusion (plus sign) before each stop word in your query.
大意就是：在搜索引擎中，stop word就是一些像the等这样的搜索引擎在索引或者检索时会主动忽略的常用词。