Definition
ABCNB is a five-letter identifier appended to an RNAV (GPS) approach chart title when an airport has multiple RNAV (GPS) approaches to the same runway. The letters are assigned alphabetically, starting from the end of the alphabet (Z, Y, X, W, V, etc.) to avoid confusion with circling-approach letter suffixes (which start from A). For example, an airport with two RNAV (GPS) approaches to Runway 18 might have charts titled RNAV (GPS) Z RWY 18 and RNAV (GPS) Y RWY 18.
Plain English
When an airport has more than one GPS approach to the same runway, each approach gets a single letter added to its name to tell them apart. The letters are picked from the end of the alphabet (Z first, then Y, then X) so they don't get mixed up with circling approaches, which use letters from the start of the alphabet (A, B, C).
Context Anchor
Seen in instrument procedure and navigation database discussions, especially when learning how fixes and waypoints are named.
Derivation
The convention uses end-of-alphabet letters (Z, Y, X...) deliberately, so that a straight-in approach named with a Z or Y is never confused with a circling-only approach named with an A, B, or C. The letters themselves carry no other meaning -- they are simply labels.
Why Pilots Care
When briefing and loading an approach, pilots must select the exact named procedure. Loading 'RNAV (GPS) Y RWY 18' when ATC cleared you for 'RNAV (GPS) Z RWY 18' loads a different procedure with potentially different waypoints, minimums, and missed approach instructions. The single letter matters.
Intuition Check
Do not try to read ABCNB as a spoken word or expand it like a normal acronym. Treat it as the name of a specific navigation point.
Example Sentence 1
Approach cleared us for the RNAV (GPS) Z RWY 27, so we made sure to load the Z procedure and not the Y, since both serve the same runway.