Core terms
Aggressor side
The side, buyer or seller, presumed to have started a trade by crossing the spread. It is inferred from the print's price against the quote; it is not a field in the trade report.
Trade reports carry price, size, time and condition codes, not which side initiated the trade. Platforms colour prints by comparing the price with the quote: at or above the ask is usually labelled a buy, at or below the bid a sell, and prints between the quotes are left uncoloured or split by which side they are closer to. Platforms differ in which quote they use, so two screens can label the same print differently.
Lee and Ready (1991)
The research version of this inference classifies a trade as buyer-initiated when its price is above the quote midpoint and seller-initiated when it is below; trades at the midpoint are classified with a tick test against the previous trade price. The inference can be wrong when the quote is stale, when a print is reported late, or when the trade was negotiated off-exchange.
What traders should verify
Record print location as an observation ("12 of 14 prints at the ask") and keep any reading of intent ("buyers are aggressive") in a separate inference column. Note which quote your platform uses to colour prints.
Do not conflate
Trader usage; there is no official definition. The label is an inference by the platform or researcher; a count of prints at the ask describes labels, not who controls the price.
Primary source
Primary sourceLee and Ready, 1991