Definition: Keyword Query Language (KQL) is used to form search criteria in SharePoint.

KQL is the default query language for building search queries. Using KQL, search term or property restrictions are passed to search service.

Some Points:

• If you create KQL from default SharePoint search front end, the length limit is 2048 characters
• If you create KQL programmatically using Query Object Model, the default length limit is 4096 characters.
• If KQL contains only operator or if KQL is empty, it is not a valid KQL.
• KQL queries are NOT case-sensitive
• KQL Operators are case-sensitive(UPPERCASE)