TITLE: What do $∞^1$, $∞^2$, or $∞^3$ mean?
QUESTION [6 upvotes]: In studying the history of Lie theory, I have seen the symbols $∞^1$, $∞^2$, and $∞^3$ (cf. Page's excellent Ordinary Differential Equations (1897), passim). What exactly do they mean?

REPLY [8 votes]: $\infty^n$ was used in the past to refer to a (typically continuous) family of solutions to a problem parametrized by $n$ parameters. This is what we call an $n$-manifold now.
The classic book Geometry and the Imagination used this notation: