What is what does o with a line through it mean?

The symbol "ø" (lowercase) or "Ø" (uppercase) is a letter used in several languages, primarily:

• Danish
• Norwegian
• Faroese

It represents a vowel sound that doesn't exist in English. Phonetically, it usually represents a close-mid front rounded vowel, similar to the sound in the German word "schön" or the French word "feu." The exact pronunciation varies slightly between the languages.

Beyond language use, "Ø" can also be used as a symbol for:

• Diameter: In technical drawings, engineering, and mathematics, "Ø" represents the diameter of a circle or a cylindrical object.
• Empty Set: It can be an alternative symbol to represent an empty set.