What language arts term refers to the rhythm of sonnet and features a series stressed unstressed syllables?

The language arts term that refers to the rhythm of a sonnet and features a pattern of stressed and unstressed syllables is meter.

Specifically, sonnets often use iambic pentameter, which is a metrical pattern consisting of five iambs per line. An iamb is a metrical foot with one unstressed syllable followed by one stressed syllable.

For example, the line "Shall I compare thee to a summer's day?" from Shakespeare's Sonnet 18 follows iambic pentameter:

* Shall (unstressed) I (stressed) com (unstressed) pare (stressed) thee (unstressed)

* to (unstressed) a (stressed) sum (unstressed) mer's (stressed) day (unstressed)

So, while meter is the general term, iambic pentameter is the specific rhythm often found in sonnets.