To me, angling is using a deeper knowledge of potentially grey poker rules/procedures to attempt to gain an advantage. There are degrees of "angling" that come close to (or that are) outright cheating or obvious theft, but much is in the eye and ethics of the beholder.
For example, a player at low blind levels in a big tournament (having many 25 and 100 chips in front of him) tosses in a 1k chip in response to a bet (without saying call) in order to get a reaction from an opponent. Some opponents might fold incorrectly assuming it's a raise, the angler knows it isn't.
When heads-up, deliberately showing a card to an opponent could be considered an angle. If I have the nut flush and show you the Ace you might assume I don't have a made hand and call with a worse hand. Angle perhaps, but also legal in most rooms. Pump-faking a bet behind the betting line is shady, but still probably would be called an angle.
Misstating the amount of a bet can be called an angle. Stating a verbal "five" then gauging an opponent's reactions could mean five hundred or five thousand in a tournament. The angler knows.
Things like deliberately mis-declaring your hand, hiding big chips, talking when there are more than 2 players in a hand aren't angles. That's cheating. Same thing with claiming physical actions were incorrectly interpreted by a dealer, which is somewhat common in casinos. For example: a partial rap of the table "I didn't check", or intentionally accidentally dropping chips "I didn't call", forward motion pseudo-calls, etc. Stating "You've got it", hoping a player tosses their hand into the muck is more cheat than angle to me. Their argument is that "you got it?" was a question.
An even more severe example that was recently shared was a player tossed a single chip across the pot in a cash game and his opponent assumed it meant a call and promptly tabled his hand. Seeing that his hand would have lost, the angler attempted to say the chip didn't signify a call, rather it was a tip for the dealer. Very much on the edge of cheating but could be called an angle by some.
Here's another cash game example that could incorrectly invite "angle" terminology. A player announces "all in" during a cash game and ends up losing the pot; however, the player didn't physically push any of his chips into the pot. The player takes his stack of $1k chips in front of him and leaves the casino giving nothing to the winner. It's an angle as the chips aren't technically in the pot and the casino never took possession of the chips to be able to legally award them to the winner. Real situation, outright theft, could also be termed an angle (incorrectly, IMO).