{multline} is a math environment provided by the amsmath package for single equations that span several lines.

The first line of the equation will be set at the left margin, the last line on the right margin, and all possible other lines will be centered. This behaviour can be changed with the \shoveleft and \shoveright commands, that take entire lines as their argument, and place them at the left, resp. right margin. A multline environment thus contains no alignment points &. The environment gets a single equation number that is placed at the first line, while the reqno option of the package shifts the equation number to the last line.

For a similar multi-line equation that shall be part of a larger equation the mathtools package provides the multlined environment.