An acephalous or headless line is a line in a poem which does not conform to its accepted metre, due to the first syllable

Syllable

A syllable is a unit of organization for a sequence of speech sounds. For example, the word water is composed of two syllables: wa and ter. A syllable is typically made up of a syllable nucleus with optional initial and final margins .Syllables are often considered the phonological "building...

's omission. Acephalous lines are usually deliberate variations in scansion

Scansion

Scansion is the act of determining and graphically representing the metrical character of a line of verse.-Overview:Systems of scansion, and the assumptions that underlie them, are so numerous and contradictory that it is often difficult to tell whether differences in scansion indicate opposed...

, but this is not always obvious. Famous poems to use such a technique include A.E. Housman's To an Athlete Dying Young

To An Athlete Dying Young

"To An Athlete Dying Young" is a poem in A.E. Housman's A Shropshire Lad . It is perhaps one of the most well-known poems pertaining to early death; in this case, that of a young man at the height of his physical glory....

. Robert Wallace

Robert Wallace

Robert Wallace was a Scottish politician. He was an electoral franchise reformer and agitator for postal service reform.He was elected to the Westminster Parliament as the member for Greenock in 1832, sitting for that constituency until 1845.Robert Wallace was the founder of the campaign for cheap...

 argues in his Meter in English that the term acephalous line seems "pejorative", as if criticising the poet's violation of scansion, but this view is not widely held among critics.