What is Opcode?

Opcode is short for operation code and refers to low-level human readable programming instructions. On Ethereum, opcode refers to the 140 unique commands that can be executed by the Ethereum Virtual Machine (EVM), the environment and rule-set in which Ethereum smart contracts and accounts exist. Writing programs in a low-level language like opcode is possible, though much more difficult than using a high-level language like Solidity. Basic examples of opcode commands are 'PUSH' and 'POP', which respectively add and remove data from the EVM.