Every 210,000 blocks, the block subsidy is cut in half in an event known as the halving. This process will continue until the block subsidy is zero, at which point miners will only collect transaction fees and Bitcoin's inflation will hit absolute zero.

Bitcoin total supply will never exceed 21 million bitcoin. This policy gives Bitcoin its absolute scarcity.

A Hard fork is a consensus change that is not backwards compatible. It requires that all nodes on a network upgrade in order to avoid splitting the network.

A cryptographic hash function is a one-way, random mathematical function. Hash functions underlie Bitcoin's security model in several ways.

Hash rate is a measure of how many hashes miners cumulatively produce per second on the Bitcoin network. The hash rate indicates how much money, energy, and computing power is being dedicated to processing transactions and securing the network.

A Hashed Time Locked Contract is a timelocked Bitcoin transaction used on the Lightning Network to route payments between parties who do not share a Lightning Channel.

A Hierarchical Deterministic Wallet is the term used to describe a wallet which uses a seed to derive many public and private keys. These keys are generated in a deterministic way, and are used to send and receive bitcoin.

Hexadecimal is an encoding scheme which uses a 16 character alphabet, including digits 0-9 and letters A-F.

HODL is an adage native to the Bitcoin community, derived simply from a misspelling of the word “hold”. It advises bitcoiners to remain stoic in the face of Bitcoin's extreme volatility.

A hot wallet is a Bitcoin wallet which connects to the internet. These wallets are more convenient for day-to-day spending, but are not as secure as cold storage.