What the division sign is called
The ÷ sign is called the obelus, from a Greek word for a roasting spit. Long before it meant division, scribes used a similar mark to flag doubtful passages in manuscripts. The Swiss mathematician Johann Rahn gave it its math meaning in 1659, in his book Teutsche Algebra.
12 ÷ 4 = 3 reads "12 divided by 4 equals 3". The sign is mostly used in primary school, on calculator keys and in simple arithmetic. In parts of Scandinavia it has also been used as a minus sign, so check the context in international documents.
÷ vs / and the fraction bar
From algebra onward, most books drop ÷ and write division as a fraction or with a slash: a/b. Keyboards, Excel and programming languages use the slash / too, as in =A1/B1. Unicode also has a dedicated division slash ∕ and a fraction slash ⁄, which fonts use to build fractions like 3⁄4; see fraction symbols for ready-made ½ and ¾.
Two more signs are easy to mix up. ⟌ is the long division bracket used in written division at school. ∣ (a thin vertical bar) means "divides" in number theory: 3 ∣ 12 says 3 divides 12 with no remainder, which reads in the opposite direction to 12 ÷ 3.
How to type ÷
On a Mac, press Option + /. On Windows, hold Alt and type 246 on the number pad (Alt + 0247 also works). In Microsoft Word, type 00F7 and press Alt + X.
On Android with Gboard, ÷ sits on the second symbols page (?123, then =\<). In LaTeX, use \div. In HTML, write ÷. Phone emoji keyboards also have a heavy ➗︎ sign. For the other basic operators, see the multiplication symbol and the full math symbols list.