In music, it is a note or pitch (or pitch class) that is not a member of the scale or mode indicated by the most recently applied key signature (https://en.wikipedia.org/wiki/Accidental_(music)).
In a more simplified definition, if a musical score is in the key of D but there is an outlier note that must be played outside the key of D, that note is marked within the bar.
Other Guitar Term
A piece of leather or fabric worn to support the guitar when standing. Straps attach to the button-like connectors on the sides of the guitar body. Some acoustic guitars that do not have the connector at the top end of the body require a small band that loops around the the headstock of the guitar just above the nut. This band allows for a guitar strap to connect to the top half of the guitar.
Standard tuning defines the string pitches as E, A, D, G, B, and E, from lowest (low E2) to highest (high E4). Standard tuning is used by most guitarists, and frequently used tunings can be understood as variations on standard tuning. There are hundreds of such tunings, often minor variants of established tunings.
Scale length refers to the distance between a guitar’s nut and its bridge. So in other words, a guitar’s scale length is determined by the gap between the two main components that seat its strings. It can therefore be thought of as the measurement of the maximum sounding length of a guitar’s strings. (https://blog.andertons.co.uk)
The guitar saddle can be a piece of bone or plastic on an acoustic guitar and a metal construct on electric guitars that is attached to the bridge and lifts the strings to the desired height transfering vibration through the bridge to the soundboard. The height of the saddle raises or lowers the action of the strings of the guitar.