In poetry, metre (meter in American English) is the basic rhythmic structure of a verse or lines in verse. Many traditional verse forms prescribe a specific verse metre, or a certain set of metres alternating in a particular order. The study of metres and forms of versification is known as prosody. (Within linguistics, "prosody" is used in a more general sense that includes not only poetical metre but also the rhythmic aspects of prose, whether formal or informal, which vary from language to language, and sometimes between poetic traditions.)

The metre of much poetry of the Western world and elsewhere is based on particular patterns of syllables of particular types. The familiar type of metre in English-language poetry is called qualitative metre, with stressed syllables coming at regular intervals (e.g. in iambic pentameter, typically every even-numbered syllable). Many Romance languages use a scheme that is somewhat similar but where the position of only one particular stressed syllable (e.g. the last) needs to be fixed. The metre of the old Germanic poetry of languages such as Old Norse and Old English was radically different, but still was based on stress patterns.

The metre (meter in the US), symbol m, is the base unit of length in the International System of Units (SI). Originally intended to be one ten-millionth of the distance from the Earth's equator to the North Pole (at sea level), its definition has been periodically refined to reflect growing knowledge of metrology. Since 1983, it has been defined as "the length of the path travelled by light in vacuum during a time interval of 1 ⁄ 299,792,458 of a second".

The first recorded proposal for a decimal-based unit of length was the universal measure unit proposed by the English philosopher John Wilkins in 1668. In 1675, the Italian scientist Tito Livio Burattini, in his work Misura Universale, used the words metro cattolico (lit. "catholic [i.e. universal] metre"), which was derived from the Greek μέτρον καθολικόν (métron katholikón), "a universal measure". This word gave rise to the French mètre which in 1797 was introduced into the English language.

In 1668, Wilkins proposed using Christopher Wren's suggestion of a pendulum with a half-period of one second to measure a standard length that Christiaan Huygens had observed to be 38 Rhineland or 39¼ English inches (997 mm) in length. In the 18th century, there were two favoured approaches to the definition of the standard unit of length. One approach followed Wilkins in defining the metre as the length of a pendulum with a half-period of one second, a 'seconds pendulum'. The other approach suggested defining the metre as one ten-millionth of the length of the Earth's meridian along a quadrant, that is the distance from the equator to the North Pole. In 1791, the French Academy of Sciences selected the meridional definition over the pendular definition because the force of gravity varies slightly over the surface of the Earth, which affects the period of a pendulum.

Poetry (from the Greek poiesis — ποίησις — with a broad meaning of a "making", seen also in such terms as "hemopoiesis"; more narrowly, the making of poetry) is a form of literary art which uses the aesthetic qualities of language to evoke meanings in addition to, or in place of, the prosaic ostensible meaning.

Poetry has a long history, dating back to the Sumerian Epic of Gilgamesh. Early poems evolved from folk songs such as the Chinese Shijing, or from a need to retell oral epics, as with the Sanskrit Vedas, Zoroastrian Gathas, and the Homeric epics, the Iliad and the Odyssey. Ancient attempts to define poetry, such as Aristotle's Poetics, focused on the uses of speech in rhetoric, drama, song and comedy. Later attempts concentrated on features such as repetition, verse form and rhyme, and emphasized the aesthetics which distinguish poetry from more objectively-informative, prosaic forms of writing. From the mid-20th century, poetry has sometimes been more generally regarded as a fundamental creative act employing language.