What is a fixed form of lyric poetry that consists fourteen lines usually written in iambic pentameter?

The fixed form of lyric poetry you're describing is called a sonnet.

Here are the key characteristics:

* 14 lines: Always has 14 lines.

* Iambic pentameter: Typically written in iambic pentameter, which is a rhythm of 10 syllables per line with an alternating unstressed/stressed pattern.

* Specific rhyme scheme: Sonnets follow a specific rhyme scheme, which varies depending on the type of sonnet.

* Shakespearean/English sonnet: ABAB CDCD EFEF GG

* Italian/Petrarchan sonnet: ABBAABBA CDECDE or ABBAABBA CDCDCD

Let me know if you'd like to know more about different types of sonnets!