Guilloché (or Guilloche) is a decorative engraving technique in which a very precise intricate repetitive pattern or design is mechanically engraved into an underlying material with fine detail. The word is French for a pattern making tool like a geometrical lathe.
The most commonly seen guilloche is the one on bank notes. Watch dials, cases and internal components like bridges and wheels are sometimes engraved in such patterns, which is always a mark of quality.
Here is a nice example of guilloche dial: