Bank notes (and other certificates) typically have patterns of fine lines, for example the pattern behind the lion here, from the Bank of England Museum:
Do these patterns have a name? What is the proper name for these patterns?
Bank notes (and other certificates) typically have patterns of fine lines, for example the pattern behind the lion here, from the Bank of England Museum:
Do these patterns have a name? What is the proper name for these patterns?
You're looking for the term Guilloché pattern:
Guilloché patterns are spirograph-like curves that frame a curve within an inner and outer envelope curve. They are used on banknotes, securities, and passports worldwide for added security against counterfeiting. — Wolfram MathWorld
More examples at Math Games.
More informally, they're known as Spirograph patterns. A Spirograph is a popular toy so this is the most well-known name.
Barmar asked a follow up question on Money SE about how this works: How does Guilloché pattern inhibit counterfeiting?