Gas on Ethereum is explained in full under Gas Fee, which covers how fees are set, paid and burned. This page gives a short definition of gas itself and explains why the word is used in two ways.
Gas is a unit of computational effort. Every operation that the Ethereum Virtual Machine performs, from adding two numbers to storing data, has a fixed gas cost set by the protocol. A simple ether transfer uses 21,000 gas, while a complex smart-contract call can use hundreds of thousands. Measuring work in gas, rather than directly in ether, keeps the cost of each operation stable even when the price of ether changes, and it stops programs from running forever, because every transaction must set a gas limit it cannot exceed.
The fee a user pays is the gas used multiplied by a price per unit, quoted in gwei, a billionth of an ether. That price, covered under gas price, rises when the network is busy. In everyday speech people often say "gas" when they mean the fee, as in "gas is high today", which is why the two terms are often mixed up. Other networks that use the Ethereum design borrow the same idea and wording.