It's a term in musical notation, or software for composition/scoring?
No, as I am using it, it is the character, symbol, or other mark defined by a single entry in a font. In music typesetting special fonts are used which contain graphical objects which are not defined in Unicode but are designed specifically for that single purpose (of course, they are allocated to Unicode code points, but in the so-called "private plane"). These graphical objects are glyphs.
There is a rudimentary standard,
SMuFL, for such fonts, but the software I use (LilyPond) has
its own - note the use of the word "glyph" on that page - which I guess is similar, because LilyPond can also use some SMuFL fonts with a modest conversion table (in fact for the scores I am doing for publication, I am using the font developed by Steinberg for their Dorico music typesetting package). The glyphs in these fonts are not always complete symbols, but may need to be combined to make actual musical symbols in the score (LilyPond calls these "grobs", for "graphical objects").
I make scores for three purposes - to make decent copies of music I want to play on the piano which is not necessarily published (e.g. anime theme songs); to make scores for the choir I sing in (often making a new edition, and also making instrumental parts when we have supporting instruments); and recently, making scores for publication of the music of a recently deceased composer (Justin Connolly) of whom my son is the executor - in which role he is trying to get all his unpublished work into print. In support of my son's efforts, I have done a piece for solo flute, which was his last composition, and is now in print; the piece for two pianos I linked, which is now in final proof (that has been published in facsimile, but that will be replaced by my typeset score); and I am now working on a piece for flute and double bass which, though older (1970s) is far more avant-garde in its notation, and will stretch what I can do with the software severely!