Gasket
What is gasket in coffee?
A gasket is the rubber or silicone ring that seals two parts of a coffee maker together, such as the top and bottom halves of a moka pot or the group head of an espresso machine. When it hardens, cracks or gets dirty, the seal fails and the brewer leaks. Replace it when that happens, often about once a year.
On a moka pot, a leak around the middle usually means a worn gasket, grounds on the rim or a pot that is not screwed tight enough. Spare gaskets are cheap and specific to each pot size.