A projection is the transformation of a set of data from one form into another.
Mathematically speaking a projection is a function that yields the same result if applied multiple times.
The term is used with this meaning in graphics when 'projecting' a 3D scene on a 2D canvas.
The same is often happening when analyzing data with many dimensions (age, gender, number of transactions, ...) and projecting it on fewer dimensions, in the simple case by just dropping dimensions or by creating new dimensions like (might commit fraud).
When dealing with typed languages the dimensions might be attributes of a class. So if I have one or multiple types with attributes (A, B, C, ...) and create from instances of those types instances of a type which has only the attributes (A, B) this is a projection too.
Another example are databases. A projection is an operation which discards some attributes of a tuple, in other words, which omits some columns of a relational database query result. For an object or document database a projection filters out some fields.
After all a database projection yields the same result if applied multiple times, too.
