Sentence:  Now, let "P", "Q", and "R" be three noncollinear points that are left fixed by an <WSD>affine</WSD> transformation "T". 
Meaning of the word marked with <WSD> tags:  Of or pertaining to a function expressible as f(\vec x)= A \vec x + \vec b  (where A is a linear transformation and \vec b is a constant), which, regarded as a transformation, maps parallel lines to parallel lines and finite points to finite points.