A transfer medical is the physical examination a signing club runs before completing a deal, covering scans of past injuries, cardiac screening, blood tests and movement assessments. It is a risk check on a long and expensive contract, and a poor result can reduce the fee or end the transfer.
Clubs are underwriting years of guaranteed wages, so the examination goes well beyond a fitness test. Imaging of old problem areas, heart screening and workload history all feed into a judgement about how many matches the player is likely to be available for across the contract.
Failing a medical rarely means being declared unfit. More often the club's medical staff flag a risk, and the deal is restructured around it: a shorter contract, a lower guaranteed fee with more of it moved into appearance-based add-ons, or an insurance arrangement.
The timing explains the deadline day scramble. A medical cannot start until the clubs have agreed enough for the player to travel, so the last hours of a window are spent on a process that ordinarily takes most of a day, which is where deal sheets come in.