1)δ Pronunciation "δ". Is the lowercase form of the Greek letter δ,

2) In mathematics, δ or δ generally represents the increment of a value.

3) On the steel plate, the steel pipe adopts the thickness of δ.

4) The capitalized "δ δ" is the code name of the discriminant for finding the root of a quadratic equation with one variable (but δ is not used).