Dr. L. Kip Wheeler, of Carson-Newman College, provides the definitions below.
IAMB: A unit or foot of poetry that consists of a lightly stressed syllable followed by a heavily stressed syllable. Some words in English naturally form iambs, such as behold, restore...[and] and so on. A line of poetry written with syllables falling in this pattern of stress are said to be in iambic meter.
[Iambic pentameter is the use of the same structure of stressed and unstressed syllables, but there are a total of five pairs of syllables, common to Shakespearean—or Elizabethan—sonnets.]
FOOT: A basic unit of meter consisting of a set number of strong stresses and light stresses.
IMPERFECT FOOT: A metrical foot [unit of meter] consisting of a single syllable, either heavily or lightly stressed.
With these definitions in mind, my understanding of an imperfect foot can be seen in the use of iambic pentameter, and William Shakespeare's sonnets are an excellent example of a poetic form based—in general—on iambic pentameter. The imperfect foot is one that does not have the traditional two-beats, but only one—which is either stressed or unstressed.
Upon reviewing the sonnet below (my favorite), we find that Shakespeare does not always adhere to lines with exactly ten-syllables. In this case, we find the imperfect foot in several lines.
A perfect foot is found in line one: which has stressed syllables located with in, -grace, for-, and, eyes. There are ten total syllables in the line.
Consider the imperfect foot in line three. The first indication that the line is unusual is found in the total number of syllables in the line. The stresses fall with: trou-, deaf, -ven, my, -less, cries. There is an extra syllable because there are eleven syllables instead of ten; the last, lone syllable is "cries."
Other lines with an imperfect foot are:
Yet in these thoughts myself almost despising, (9)
Like to the lark at break of day arising (11)
In both lines, note there are a total of eleven syllables in each line. The rest of the sonnet presents lines with ten syllables each. Generally, Shakespeare's only other variation is the use in some lines of nine syllables only, but there are none in Sonnet 29.
When, in disgrace with fortune and men's eyes,
I all alone beweep my outcast state
And trouble deaf heaven with my bootless cries
And look upon myself and curse my fate,
Wishing me like to one more rich in hope, 5
Featured like him, like him with friends possess'd,
Desiring this man's art and that man's scope,
With what I most enjoy contented least;
Yet in these thoughts myself almost despising,
Haply I think on thee, and then my state, 10
Like to the lark at break of day arising
From sullen earth, sings hymns at heaven's gate;
For thy sweet love remember'd such wealth brings
That then I scorn to change my state with kings.