Product activation
License enforcement in which software must be approved by the vendor's server and bound to a specific machine before it fully works.
A license key on its own only proved that someone had seen a key. Activation adds a second step: the program derives an installation ID from the hardware, sends it to the vendor and receives a code that only fits that machine. Windows XP made the model famous in 2001, with a 30-day grace period and a phone line for the offline. Swap too many components and you activate again. Businesses usually sidestepped it through volume licensing.
The downside appears when the server goes dark. When Adobe shut down activation for Creative Suite 2 around the turn of 2012–2013, legally purchased copies could no longer be installed, and the company ended up publishing serial numbers that skipped activation. Pirates with their keygens were, naturally, unaffected.