Horizontal blank (HBlank)
The short pause as the beam jumps from the end of one line to the start of the next — the window for per-line tricks.
Between each scanline the beam is blanked briefly while it retraces to the left edge. Some systems could fire an interrupt right then (the HBlank interrupt), and in that gap you could change hardware registers between lines.
It is the engine behind half the graphics tricks of the 1980s and 90s: per-line palette swaps, scroll values that vary down the screen (parallax), the SNES Mode 7, and the Amiga's copper raster bars. "Racing the beam" was, in practice, about what you could finish during each HBlank.