Scope: The ideal engine would be able to search every document on the Internet

Speed: Results would be available immediately

Currency: All the information would be kept completely up-to-date

Recall: We could always find every document relevant to our query

Precision: There would be no irrelevant documents in our result set

Ranking: The most relevant results would come first, and the ones furthest afield would come last

Of course, our mundane search engines have a way to go before reaching the Platonic ideal.

via Patterns in Unstructured Data
Discovery, Aggregation, and Visualization, by

National Institute for Technology and Liberal Education .