A chip transaction is the everyday name for an EMV transaction, a card payment in which the merchant's terminal communicates with the microchip on the card, and that entry explains the process in detail.
When the card is inserted into a terminal, the EMV chip and terminal agree on the payment, check the cardholder, often by PIN, and the chip produces a cryptogram unique to that transaction. The issuer verifies the cryptogram during authorisation, which makes cloned cards much less useful than with the old magnetic stripe. Contactless payments use the same chip technology through a radio link, so they are also chip-based, although in everyday speech "chip transaction" usually means inserting the card.
Industry documents prefer "EMV transaction" because EMV is the formal standard, maintained by EMVCo. If the chip cannot be read and the terminal falls back to the stripe, the payment becomes a fallback transaction, which carries higher fraud risk and may shift liability under card network rules.