Aggressive Participants are those traders who are willing to trade at the best available market price at that time. They are not willing to wait for a price to be filled at a specific level and end up Crossing the Market and “Lifting the Offer“ when buying and “Hitting The Bid” when selling.
The level of ‘aggressiveness’ of these participants can be measured by Volume Delta. The actions of Aggressive Participants can often provide clues to traders as to what might be likely to happen next over the short term.
See also Passive Participants, Volume Delta, Reloading.