Off-chain is a term used to describe any data that is not registered on the Bitcoin blockchain.
On-chain is a term used to describe any data that is registered on the Bitcoin blockchain. On-chain data is almost exclusively comprised of Bitcoin transactions.
An operation code, or opcode, is a basic command of some computer languages. Bitcoin’s scripting language, called Script, has its own set of opcodes.
Open-source describes a project whose source code is publicly available and licensed such that individuals can use and alter the source code freely.
An orphan block is a valid block that was excluded from the blockchain because another block was mined at the same height and was part of the longer chain.