If you can drive your car down the highway and then go off road and ford a few rivers, too, you've got an amphibious vehicle — something that works on land and in the water, too.
Amphibious also describes animals that can live on land or in the water — like frogs and toads — and comes from the Greek roots amphi "both" and bios "life." Military operations where both your army and your navy are involved — one attacking by land and the other by sea — are called amphibious assaults or amphibious operations.
