What is an anion? Definition:

As anion (Ancient Greek ana = up; ion = migratory) is an electrically negatively charged ion. The name is derived from the direction in which negatively charged ions move during electrolysis: up to the anode.
An anion can arise from a neutrally charged atom by taking up one or more electrons. Electrons are negatively charged elementary particles. As a result, the atom has an excess of electrons or negative charge surplus.
The counterpart to an anion is the positively charged cation.