EVM
Ethereum's virtual machine, and the de facto standard that many other chains copy so the same contracts can run on them.
The EVM is a stack machine with 256-bit words, and contracts written in Solidity or Vyper are compiled to its bytecode, with each opcode carrying a gas cost that reflects the work and storage it imposes. Its importance comes less from its technical design than from its network effects: wallets, block explorers, libraries, auditing tools and developer knowledge all target it, so a new chain that runs EVM bytecode inherits an entire ecosystem. Compatibility is a spectrum, and chains vary in gas schedules, available precompiles, opcode behavior, and block timing, so identical bytecode does not always behave identically. Readers meet the term whenever a chain is described as EVM-compatible or a rollup as a zkEVM.
In practice
Deploying the same contract bytecode to several EVM chains is routine, but differences in gas costs or block timing can change how it behaves in practice.
The common misunderstanding
EVM compatible does not mean identical; differing gas schedules, precompiles or timing can make the same contract behave differently on another chain.