What does the term "projection" refer to in an SQL query (or view definition)
In Relational algebra, projection means collecting a subset of columns for use in operations, i.e. a projection is the list of columns selected.

In a query optimiser step, the projection will manifest itself as a buffer or spool area of some description containing a subset of the columns from the underlying table or operator, or a logical view based on those columns that is used by subsequent operations.

In a view, a projection equates to the list of columns selected in the query underlying the view.