Why is "Lookup" colored as a function reserved word in SQL Server?

In SSMS 2208, the identifier "Lookup" is colored hot pink as if it were a function (same color as, say, "Power" or "Convert"). Why? I cannot find it in the [official list of reserved words](http://msdn.microsoft.com/en-us/library/ms189822.aspx) . Searches on the web seem useless as there are an awful lot of "lookup" terms out there that have nothing to do with my question.