[JimRed]

Tic-tac-toe? I thought it was a symbol for the word “number” as in #1.

[Larry Lucido]

It used to be. Then with push-button phones, the phone company called it the “pound” sign. Maybe because you have to “pound” the button. :-)

Now I’m seeing it everywhere for some reason.

[zeugma]

Tic-tac-toe? I thought it was a symbol for the word “number” as in #1.

Me, I'd say it was an octothorp.

From Wikipedia:

Octothorp, octothorpe, octathorp, octatherp

Used by Bell Labs engineers by 1968.^[6] Lauren Asplund says that he and a colleague were the source of octothorp at AT&T engineering in New York in 1964. The Merriam-Webster New Book of Word Histories, 1991, has a long article that is consistent with Doug Kerr's essay,^[7] in that it says "octotherp" was the original spelling, and that the word arose in the 1960s among telephone engineers as a joke. The first appearance of "octothorp" in a US patent is in a 1973 filing which also refers to the six-pointed asterisk (✻) used on telephone buttons as a "sextile".^[8]