A meter in poetry, consisting of lines with five feet (hence "pentameter") in which the iamb is the dominant foot (hence "Iambic"). Iambic rhythms are quite easy to write in English and iambic pentameter is among the most common metrical forms in English poetry. Like the rest of the meters it has its origins in Greek poetry.
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: