Diameter sign
It has no Windows Alt code: copy it from this page, or use its HTML entity or Unicode number.
Copy it
About the diameter sign
Engineering drawings put ⌀ before a measurement to show that it is a diameter: ⌀ 55 mm is a hole or shaft 55 mm across. It looks very like the Danish and Norwegian letter ø and the empty set sign ∅, but each is a separate character with an unrelated meaning.