Bid-side / ask-side
Where a trade printed relative to the quote. Prints at or near the ask are often read as buyer-initiated, and prints near the bid as seller-initiated.
Every option has a bid, the price buyers are posting, and an ask, the price sellers are posting. When a trade prints at or above the ask, a scanner usually labels it ask-side and infers that the buyer initiated it by paying the seller’s price. A print at or below the bid is labeled bid-side and read as seller-initiated.
The inference can be wrong. Quotes move between the order and the print, complex orders execute against several legs at once, and a print at the ask may close an existing short position.
Illustrative example (hypothetical numbers, not a real trade): a put is quoted $2.00 bid and $2.20 ask. A print of 300 contracts at $2.20 is labeled ask-side, and a print of 300 at $2.00 is labeled bid-side. The same 300 contracts at $2.10 would be a midpoint print.