Anion (−) and cation (+) indicate the net electric charge on an ion. An ion that has more electrons than protons, giving it a net negative charge, is named an anion, and a minus indication "Anion (−)" indicates the negative charge. With a cation it is just the opposite: it has fewer electrons than protons, giving it a net positive charge, hence the indication "Cation (+)".
Since the electric charge on a proton is equal in magnitude to the charge on an electron, the net electric charge on an ion is equal to the number of protons in the ion minus the number of electrons.
An  (−) ( , from the Greek word ἄνω (ánō), meaning "up") is an ion with more electrons than protons, giving it a net negative charge (since electrons are negatively charged and protons are positively charged).
A  (+) ( , from the Greek word κάτω (káto), meaning "down") is an ion with fewer electrons than protons, giving it a positive charge.
There are additional names used for ions with multiple charges. For example, an ion with a −2 charge is known as a dianion and an ion with a +2 charge is known as a dication. A zwitterion is a neutral molecule with positive and negative charges at different locations within that molecule.
Cations and anions are measured by their ionic radius and they differ in relative size: "Cations are small, most of them less than 10 m (10 cm) in radius. But most anions are large, as is the most common Earth anion, oxygen. From this fact it is apparent that most of the space of a crystal is occupied by the anion and that the cations fit into the spaces between them."
The terms anion and cation (for ions that respectively travel to the anode and cathode during electrolysis) were introduced by Michael Faraday in 1834 following his consultation with William Whewell.