The octopus (pronounced /ˈɒktəpʊs/, from Greek ὀκτάπους (oktapous), "eight-footed", with plural forms: octopuses /ˈɒktəpʊsɪz/, octopi /ˈɒktəpaɪ/, or octopodes /ɒkˈtɒpədiːz/; see below) is a cephalopod of the order Octopoda. The octopus inhabits many diverse regions of the ocean, especially coral reefs. The term may also be used to refer only to those creatures in the genus Octopus.

In the larger sense, there are around 300 recognized octopus species, which is over one-third of the total number of known cephalopod species. An octopus has eight arms, which trail behind it as it swims. Most octopuses have no internal or external skeleton, allowing them to squeeze through tight places.

An octopus has a hard beak, with its mouth at the center point of the arms. Octopuses are highly intelligent, probably the most intelligent of all invertebrates. For defense against predators, they hide, flee quickly, expel ink, or use color-changing camouflage.

Octopuses are bilaterally symmetrical, like other cephalopods, with two eyes and four pairs of arms. All octopuses are venomous, but only the small blue-ringed octopuses are deadly to humans. Octopuses are characterized by their eight arms, usually bearing suction cups.

The arms of octopuses are often distinguished from the pair of feeding tentacles found in squid and cuttlefish. Both types of limbs are muscular hydrostats. Unlike most other cephalopods, the majority of octopuses – those in the suborder most commonly known, Incirrina – have almost entirely soft bodies with no internal skeleton.

They have neither a protective outer shell like the nautilus, nor any vestige of an internal shell or bones, like cuttlefish or squid. A beak, similar in shape to a parrot's beak, is the only hard part of their body. This enables them to squeeze through very narrow slits between underwater rocks, which is very helpful when they are fleeing from morays or other predatory fish.

The octopuses in the less familiar Cirrina suborder have two fins and an internal shell, generally reducing their ability to squeeze into small spaces.

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