It always used to mean “covering the spread,” i.e. absolutely tiny trades. In professional trading circles, it still does.
In beginners’ forums online (but nowhere else, really) so many people use it just to mean “fast-ish, small-ish trades” that it’s almost lost its original meaning.
People here talk about “scalping 5-minute charts” and sometimes even “scalping 15-minute charts”!! That’s all rubbish. Scalpers aren’t using timed charts at all, those are much too slow. They’re using tick charts.
Also you can’t “scalp” CFDs anyway, because there isn’t a market to scalp. You’d be scalping your own “broker” (actually the counterparty) who obviously wouldn’t tolerate it for very long, if you were any good at it, if you think about it?! (In reality, nobody is any good at it, for good and reliable reasons, and “brokers” make a lot of profit out of all the people trying!).
“Day trading” is a term that’s a hangover from stock trading. It just means “closing your position in time to avoid settlement,” i.e. before the end of the business day. In forex and in derivative-trading, the correct term now is “intraday trading,” i.e. “within the day.” Forex doesn’t close overnight (only at weekends) so it’s an out-of-context expression anyway.
For anything that involves indicators, don’t look at anything faster than daily charts, because almost all indicators were designed from daily charts, to work on daily charts (a point usually overlooked by beginners, promoters, self-promoters and people selling services/products/information on Youtube!). You might find indicators of some value on 8-hour charts (those can be used more or less to follow forex “sessions”, which does at least have some underlying logic) but the idea of trying to use multi-indicator “systems” on hourly or 15-minute charts is, to put it very politely, “rather optimistic,” though is this forum many will swear that it “works” simply because they’re so desperate for it to “work” that they have come to believe it, as hope has triumphed over experience. (In reality, of course, it’s actually traders that “work,” not “systems”.)
I hope this helps.