I've come across the term annular ring in parentheses following washer in my calculus textbook: "has the shape of a washer (an annular ring)". The definition of the word "annular" in Merriam Webster is "of, relating to, or forming a ring". With that, the term annular ring seems redundant to me. A Google search of the phrase returns around 946,000 results, and leads me to question my conclusion.
From what I have found, it appears that annulus would have been a more appropriate choice by the author of this textbook, as that is what the shape in question represents.
Is this usage redundant, or have I missed a subtlety in its usage? For example, does mathematics (especially in calculus and/or geometry) provide for non-annular rings? If so, what distinguishes a ring that is annular from a ring that is not annular but still qualifies as a ring?