There isn’t one in English. However, German and Dutch speakers sometimes misuse the technical one for “diameter” because their word is the same.
The technical symbol for a diameter is a circle with a diagonal slash through it. But a doorsnee or Durchschnitt can mean not only a cross-section or diameter, but also an average. Confused the hell out of me until the dual meaning twigged… my car telling me it had a diameter of 37 km/h?
- keep the O-slash symbol for diameters
- stick to abbreviated forms for an average (such as avg.)
But who knows? People will assume that Porsche can’t be wrong and will start copying that usage. And if people keep seeing it, it will gain currency.
Prevalence: low. Turns up occasionally where space is at a premium, for instance in column headers.
Frequency: moderate. Quite high even, in the rare cases where applicable.
Native: no. Quite confusing, actually.