Anions are best defined as:

Anions are accurately described as negative ions that are formed by the gain of electrons. When an atom gains one or more electrons, it acquires a negative charge due to the imbalance between the number of protons (which are positively charged) and the number of electrons (which are negatively charged). This electron gain transforms the atom into an anion, making it negatively charged. For example, when a chlorine atom gains an electron, it becomes a chloride ion, which is an anion with a negative charge of -1.

In the context of the other explanations, positively charged ions are referred to as cations. Neutral ions, on the other hand, would not be classified as anions since they have no net charge. Similarly, an atom with no charge cannot be defined as an anion because it does not involve the gain or loss of electrons to create a charged particle. Thus, the definition of anions aligns perfectly with the idea that these ions are specifically formed by the gain of electrons, leading to a negative charge.