What's the difference between a fund and a security?

Question

I have always used the terms synonymously but I think there is technically a different between the two.

For example, given two seemingly similar assets, one is classified a security and the other a fund:

╔═══════╦════════════════════╦═══════════╦══════════╦════════════════╗
║ Asset ║    Description     ║   Type    ║ Holdings ║ Classification ║
╠═══════╬════════════════════╬═══════════╬══════════╬════════════════╣
║ ZAG   ║ BMO Aggregate Bond ║ Index ETF ║     1203 ║ FI Security    ║
║ ZDB   ║ BMO Discount       ║ Index ETF ║      261 ║ FI Fund        ║
╚═══════╩════════════════════╩═══════════╩══════════╩════════════════╝

Answer 1

https://www.investopedia.com/terms/f/fund.asp

A fund is a pool of money that is allocated for a specific purpose. A fund can be established for any purpose whatsoever, whether it is a city government setting aside money to build a new civic center, a college setting aside money to award a scholarship, or an insurance company setting aside money to pay its customers’ claims.

That's not some "special" definition of fund. That is the (noun) definition of fund.

https://www.investopedia.com/terms/s/security.asp

The term "security" is a fungible, negotiable financial instrument that holds some type of monetary value. It represents an ownership position in a publicly-traded corporation—via stock—a creditor relationship with a governmental body or a corporation—represented by owning that entity's bond—or rights to ownership as represented by an option.

Therefore, in this sense, a fund is a pile of money that has been used to buy a bunch of securities for the purpose of diversification.