Polygon
Financial Instrument
A blockchain built on top of Ethereum, offering faster and less expensive transactions, used here for testnet deployment.
Mentioned in 3 videos
Videos Mentioning Polygon

Vitalik: Ethereum, Part 1
Naval

Build a WEB3 app to mint unlimited NFTs… But should you?
Fireship
A blockchain platform built on top of Ethereum, offering faster and less expensive transactions, used for deploying the NFT application to its Mumbai testnet.

Vitalik Buterin: Ethereum 2.0 | Lex Fridman Podcast #188
Lex Fridman
A Layer 2 network that is a sidechain to Ethereum, which makes pragmatic trade-offs for faster scalability, though with a different security model than rollups.