A poetic technique (actually a metrical form) developed by Gerard Manley Hopkins where the number of syllables in a foot may vary (usually between one and four), but each foot will have only one stressed syllable. As a result, you may use a seven-syllable line followed by a twelve-syllable line, but both lines would have the same number of stressed syllables (say, five). The technique makes it possible for stressed syllables to run together freely in a syncopated fashion, whereas in traditional metres they are generally separated.
Hopkins considered this the poetic form closest to the "natural rhythm of speech", and often used accents to indicate where stress should be placed while reading the poem. This can be seen in his poem Binsey Poplars.
Sprung rhythm affords the poet far more freedom than the dominant metres (e.g. the iambic / trochaic etc.), but its imitation of 'natural' speech limits its application somewhat, and Hopkins remains the form's primary exponent.
Hopkins on sprung rhythm: