What does this at sign mean?

The Schläfli symbol is a part of geometry, used to represent the shape of polyhedra.

This reaction occurs through a chemical process.

He is an excellent chemist who made many discoveries.

He is an expert in petrochemistry.

It's difficult to decipher this chemical equation.

Understanding chemical equations is difficult.

He is an expert in chemical analysis.

We are growing vegetables without using chemical fertilizers.

Understanding chemical equilibrium is very important in understanding chemical reactions.