What is the proper name for a circle with a line through it?

The general prohibition sign (official name, according to ISO 7010), also known as a no symbol, no sign, circle-backslash symbol, nay, interdictory circle or universal no, is a red circle with a red diagonal line through it (running from top left to bottom right), completely enclosing a pictogram to indicate something is not permitted. The symbol is sometimes black instead of red when color is not available.

ISO 7010 is an extension of ISO 3864-1 set in 2002.

ISO 3864-1 sets the rules for the color and shape of safety signage, as well as regulating the incorporation of text according to viewing distance and sign size. The range of color and shape defined in this standard for the "prohibition symbol" or "no symbol" is defined as "a prohibition surround shape (a red circular band with a red slash going from the upper left to the lower right) over the top of a black graphical symbol."

^{Source: Wikipedia}

According to ISO 7010, it's called a Prohibition sign (and a General prohibition sign if nothing is shown behind it). Wikipedia lists a few other common names in the article 'No symbol'.

The corresponding emoji, 🚫, is called the No Entry Sign, even though it

does not look like the no-entry road sign that one might expect.