A legend is an inscription on a keycap.
Traditionally alphanumeric and control legends were centred on the keycaps.
The Shift key was the only modifier. Shifted symbol on top of the un-shifted symbol.
It was also common that text was in capitals.
To have room for all these symbols on the same key, legends were placed in the four corners of the top of the key caps. This scheme was pioneered by Digital Equipment with the much influential DEC LK201. On the DEC LK201, modifier legends were top/left. Most other major computer manufacturers mimicked DEC's scheme. One of these was IBM which used the scheme for its IBM Enhanced Keyboard, with the difference that modifier legends were middle/left-aligned, not top.
Alphanumeric characters continued to be in capitals, placed in the upper left corner of the key.
Side-printing or front-printing indicates that a legend is printed on the front side of a keycap.
Traditionally, only legends for special symbols or special functions used to be side-printed. For instance, the Sys Rq and Break legends on PC keyboards use to be side-printed.
The Commodore 64 had side-printed graphic symbols.
All symbols ("ninja")
The Filco Majestouch 2 Ninja was one of the first keyboards to have all legends printed on the front side rather than on the top. Filco/Diatec has been accused of doing it this way because of the normal legends having been accused of being worn off too easily.
The term ninja has been picked up by keyboard enthusiasts and used to denote other keyboards and keycaps with all side-printed legends, although the term "side-printed" is just as common.
Other keyboard manufacturers have start on doing side-printing, but used other names for it. Several side-printed keycap sets have also been made by enthusiast groups and by small keycap companies targeted to enthusiasts.