When liquid or gas slowly leaks through tiny holes or gaps in a container or barrier, it's called seepage. The seepage of rain water through the foundation of your house can result in a very damp basement.
Seepage is the process of something seeping, or leaking extremely slowly. You usually can't see seepage happening, the way you'd observe a leak from a faucet. Water seepage from an upstairs bathroom can gradually stain the ceiling of the room below, and the seepage of gas from your stove can be very dangerous to breathe. We can trace this word back to a root meaning "to pour out, drip, or trickle."