Cryptocurrency Glossary / dictionary

Halving refers to a reduction by one half in the amount of reward earned for mining a cryptocurrency. All bitcoin-based currencies (such as bitcoin and bitcoin cash, etc.) offer a fixed reward to miners. But these rewards get cut in half approximately every 4 years. More specifically, it happens after every 210,000 blocks are mined. As of 2021, the current reward for mining a block of transactions is 6.25 coins. The halving will continue until the maximum number of coins have been mined. In bitcoin's case, this will be when 21 million have been created. When a halving occurs, miners earn fewer bitcoins, but they will make up for that (if) each coin's price increases. Plus, they can earn transaction fees for their mining. Note that halving is not a part of all cryptocurrencies.

A "hash" (function) is a mathematical formula (an equation/algorithm) that accepts any arbitrary amount of input data (such as a short string of letters and/or number, a long text file, a digital photo file, etc...) and computes a fixed-length output value. No matter how long the input data is, the output is always a fixed length. A hash value is a shortened representation of the original message. A "good" hash algorithm for cryptographic purposes:

• Is "one-way", i.e., it can't be reversed to determine the input. (This actually depends on the amount of computational power applied to the attempted reversal, plus the quality of the hash function).
• No two sets of input data will result in the same hash output value. Otherwise, this would be considered a "collision" (which is a bad thing).
• Is "deterministic" since the same output value will always be computed given the same input data.
• Does not take much computational power to generate the output.
• Even a very small change in the input value will have a dramatic change in the output value. (This is so an attacker can't determine a pattern of outputs and use that to guess inputs)

The purpose of the hash is a security technique to prove that you own the input data (since only you can hash the input to produce the related output). A hash function can also be used as a data integrity check to be sure that the input data has not been changed, either accidentally or maliciously, since the short output can be more easily compared then the long input. The output of a hash function can also be called: "data signature", "message digest", "hash", "hash value", "digest", "checksum", "digital fingerprint". Some currently common hash functions are MD5 and SHA-1, SHA-2, SHA-3. (Note that some of these have already been demonstrated to have potential vulnerabilities)  

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A Hidden Wallet is a wallet feature which allows you to create two wallets on one device. One can be considered a “dummy” wallet which you can load with a few bitcoins, and the other wallet is the “hidden wallet”, which can contain your real fortune. If a thief forces you to reveal your secret codes then you only have to reveal your dummy wallet codes.  The thief will think that he got what he came for and you will be left with your fortune intact.  One specific wallet that has this feature is the Trezor Hardware wallet.

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HODL means "Hang On For Dear Life". It refers to the philosophy that cryptocurrency investors should not sell their currency in a panic if there is some sudden price drop. They should "hold on" for long term big profits. The bitcoin urban legend is that "HODL" is actually someone's misspelling of the word "hold" in a cryptocurrency forum. As in "Hold On!!!". After that, the crypto community adopted it, and the term took on its own meaning for investors who believe in the future of cryptocurrency and don't lose faith when the price goes down. A "Hodler" is someone who follows the HODL philosophy and doesn't sell.