A metrical foot consisting of an unaccented syllable (noted by "x") and an accented or stressed one (noted by "/"). For example:

               x    /
               a   wall
               x    /

Iambic is the most common metrical measure in English verse. A line from Christopher Marlow serves to illustrate:

            x    /     x    /    x  /     x   /
          Come live | with me | and be | my love.

Quite often an iambic pentameter (an iambic line of 10 syllables--five feet consisting of two syllables each) is slightly irregular, for instance the opening line of Frost's "Mending Wall":

       x    /      x    /    x    /      x   /     x  /
     Something | there is | that does | n't love | a wall.
  • See an overview of meter.

    A note on the source.