On the ZX81 and ZX Spectrum, they called them "User-Defined Graphics" or UDGs. I think I wrote a pixel-flipping grid font editor for characters on every computer I had access to! The ZX81, the ZX Spectrum, the Apple II (once I noticed that one of the games I had (Taipan?) hooked the text output routines into a hires character generator), and eventually various x86 programming environments, although there you were editing bitmaps, not changing the character generator source. :-)