The word ode - pronounced exactly as the English word 'owed', IPA: /əʊd/ - comes from the Greek word ōdē (ᾠδή) which means: song. It is used to describe, or appears in the title of, poems of a number of different types. In the context of ancient Greek literature the word ode is applied to two rather different types of poem.
It is applied to the poems of such lyric poets as Alcaeus (7th century BCE) and Sappho (6th century BCE). These are short poems expressive of the personal feelings of the poet. Alcaeus and Sappho use a variety of metres, but their poems are typically written in short stanzas (of between two and four lines) with each stanza in a poem written to the same metrical scheme. This type of short poem was imitated by the Roman poets Catullus (84-54 BCE) and Horace (65-8 BCE). Odes of this kind are sometimes called Sapphic odes or Horatian odes, and are paradigm examples of lyric poetry.
The word ode is also applied to the poems of the Greek poet Pindar (518-438 BCE). Pindar's odes celebrate public events: they are typically addressed to an athlete who has won a victory in one or other of the Greek Games (such as the Olympic Games or the Nemean Games), and they commemorate or celebrate this victory. (For this reason they are sometimes referred to as epinikia, i.e., songs of victory or triumphal odes.) They employ an exalted or elevated style and are constructed according to a distinctive pattern. A Pindaric ode is always composed of sets of three stanzas, each stanza being fairly long - stanzas of 14 , 15, or 16 lines are not uncommon. Each set of three stanzas consists of a strophe, an antistrophe, and an epode, the strophe and antistrophe having the same metrical scheme and the epode having a different one. This pattern of strophe, antistrophe, and epode is maintained throughout the ode.
In the context of English literature what justifies the description of a poem as an ode - or the appearance of the word ode in its title - is that it is modelled on or bears some resemblance to a Pindaric ode - though the resemblance is almost always only partial and is sometimes rather slight.
Of course, English odes do not celebrate athletic victories, but they are celebratory and they are typically addressed to a non-human subject - as with Ode to Duty by William Wordsworth (1770-1850), Ode to a Nightingale, Ode to a Grecian Urn, and Ode to Psyche, by John Keats (1795-1821), and Ode to the West Wind by Percy Bysshe Shelley (1792-1822). There is also usually a certain elevation of tone or style. Further, although few English poets have tried to imitate exactly Pindar's complex metrical schemes and the distinctive pattern of strophe, antistrophe, and epode (which in any case were not properly understood by the first English poets to write odes), most English odes do reflect the formal features of a Pindaric ode to some extent - if only in having relatively long stanzas with a complex metrical scheme or in having long stanzas of different lengths and different metrical schemes. (A good example of this latter is Wordsworth's Ode - Intimations of Immortality from Recollections of Early Childhood, whose ten stanzas range between 8 and 39 lines in length and have an average length of 20 lines.)
In the light of the classical origins of the ode as a poetic form two English odes have a particular interest. The first is The Progress of Poesy by Thomas Gray (1716-1771). This poem, which is subtitled A Pindaric Ode, replicates in its sequence of long stanzas the distinctive Pindaric pattern of strophe, antistrophe, and epode, and is (as its subtitle implies) a very good example of a Pindaric ode in English. The second poem is An Horatian Ode upon Cromwell's Return from Ireland by Andrew Marvell (1621-1678). This poem resembles a Pindaric ode in subject and style - it celebrates a public event - but models itself, so far as the metrical scheme is concerned, on the Sapphic or Horatian ode: it is written in four-line stanzas which consist of a pair of rhyming iambic tetrameters followed by a pair of rhyming iambic trimeters.
Needless to add, a poem may clearly be an ode without the word ode appearing in its title. This is the case with one of the most famous odes in English, A Song for St. Cecilia's Day, 1687 by John Dryden (1631-1700). This poem is an ode to music - St. Cecilia is the patron saint of music - and has been memorably set to music by, amongst others, Henry Purcell (1659-1695) and George Frideric Handel (1685-1759).