margin trading overwhelmingly refers to paying a percentage as a collateral, not a stop-gap loan to cover ACH transfer delays. Yes you have to have a margin account to trade with immediately deposited funds (thanks ACH), but that's not the typical usage of the phrase and as such isn't what you'll find being described when you look up the term.

Particularly since you can trade with unsettled funds using a cash account in some circumstances, see situation #3 here: https://www.sec.gov/oiea/investor-alerts-and-bulletins/ib_ca...