Summon

Summoning is the process by which a magic-user calls forth, or "summons" a creature to do their bidding.

Storyline

Prerevisionist

Summoning in prerevisionist material was the literal movement of a being to the presence and control of the mage or planeswalker summoning it. Some summoned creatures agreed to be servants of the spellcaster, while others were enslaved by the spell known as the geas, to be used in magical combat. If the wizard doing the summoning was victorious, the summoned creatures were usually returned to their places of origin; but if the wizard failed, the surviving summoned creatures would be stranded where they were summoned.

Revisionist

The modern idea of summoning involves both the prerevisionist way and the creation of a faux entity based on the concept of that creature, which is pulled from the aether. These summoned creatures have no will of their own, and vanish when no longer needed. This concept is described in The Eternal Ice by Lim-Dûl to Jodah. Though Jodah is not in fact summoned, as Lim-Dûl suggests, the explanation is still valid and continues to be the best flavor related description of the summoning process.

The summoning of unique, legendary individuals is similar to this, but based on the concept of a specific individual and not simply off of a group. It is also described in The Eternal Ice.

Planeswalkers can only summon creatures they’ve personally interacted with, so that obviously limits them to creatures from planes they’ve visited. Ordinary wizards can therefor only summon creatures from their own plane.

In-game

A Summon spell, Summon Creature, or Summon is an obsolete card type which had the form of "Summon <creature type>" in the type line of a card (where <creature> would be the current creature subtype). In the Sixth Edition rules changes, it was replaced with the card type "creature".

If a spell was a Summon Spell it would indicate what class of creature [was] being summoned.

Summoning was the act of casting a card. Once the card was played it was not longer considered a spell, but represented a permanent creature of the mentioned type. After it was cast, that creature would have summoning sickness.

From the glossary of the Comprehensive Rules (June 7, 2024—Modern Horizons 3)

Summon (Obsolete)

Older creature cards were printed with “Summon [creature type]” on their type lines. All such cards have received errata in the Oracle card reference to say “Creature — [creature type].” (Many of these cards’ creature types have also been updated.) See Creature.