The phrase "which is" is used to introduce additional information or clarification about a subject. It often acts as a way to specify or explain something further related to the noun it follows. It can help in giving more detail or context to a statement.
The phrase "which is" combines the Old English word "hwilc" (meaning "which") and the verb "is," originating from the Old English "eom," which means "to be." The combination has been used in English literature for centuries to provide specificity.