Definition
Iambic Pentameter is a type of meter in poetry consisting of lines with five feet (hence "pentameter") in which the iamb is the dominant foot (hence "Iambic"). Iambic refers to the type of poetic foot being used. Pentameter refers to the line length in number of feet.
This metrical unit consists of one short syllable followed by a long syllable. It uses a set meter to define the number of syllables and the sequence of stresses that the poem will follow.
da-DUM da-DUM da-DUM da-DUM da-DUM
weak STRONG - weak STRONG - weak STRONG - weak STRONG - weak STRONG
Was-THIS the-FACE that-LAUNCH'D a-THOU sand-SHIPS
The rules for iambic pentameter are that each line can only have ten syllables, and these are five pairs of alternating stressed and unstressed syllables. It creates a formula for the rhythm, and rise and fall of words within poetry, usually producing a musical quality.
It is easier to understand this if you read a poem using iambic pentameter, but it will generally sound something like this: ba-BUM / ba-BUM / ba-BUM / ba-BUM / ba-BUM. Each of these sets are known as an iambus, and create a sequence of beats.
Different meters have been used in poetry, but the iambic pentameter seems to be the most well-known and successful. William Shakespeare was famous for his use of Iambic Pentameter, and the majority of his poetry and sonnets follow this meter.
Example
Here is an example from William Shakespeare's Sonnet XVIII:
Shall I compare thee to a summer's day?
Thou art more lovely and more temperate:
Rough winds do shake the darling buds of May,
And summer's lease hath all too short a date:
Origin and History
Like the rest of the meters, Iambic Pentameter has its origins in Greek poetry.
Example
da-DUM da-DUM da-DUM da-DUM da-DUM (weak STRONG / weak STRONG / weak STRONG / weak STRONG / weak STRONG) Was-THIS the-FACE that-LAUNCH'D a-THOU sand-SHIPS
Here is an example from William Shakespeare's Sonnet XVIII:
Shall I compare thee to a summer's day?
Thou art more lovely and more temperate:
Rough winds do shake the darling buds of May,
And summer's lease hath all too short a date: