https://www.coindesk.com/learn/2021/02/17/what-is-a-flash-loan/TLDR: It's a contract of contracts, one of which is an uncollaterized loan typically followed by a sequence of trades which all must occur atomically (in one transaction) on the blockchain.
More technical breakdown from aave, the leading choice for flash loans:
https://docs.aave.com/faq/ 
