## Holochain Glossary

Browse the glossary using this index

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### M

#### Merkle Proof

Mathematical proof to show that any small part of data is part of a much larger set of data and has not been tampered with. The proof does not require access to the entire large data set to work. The proof relies on a data structure called a Merkle Tree. (see Merkle Tree).

#### Merkle Tree

A hash tree that allows efficient and secure verification of the contents of large data structures. Something like a blockchain combined with a binary search. e.g. A user of a Merkle tree with a publicly known and trusted root can ask for a Merkle Proof to verify any value in the tree is correct. (see Merkle Proof).

The idea that stored data only increases, never decreases. This means all data points are kept then aggregated (e.g. using a max function) rather than updated in place. In distributed systems, it is very hard to synchronize the removal of data, so we keep a record of the existence of the data, and mark it retracted (or expired, or flagged as invalid). A retraction is not a deletion of the original data but an addition of new data that asserts the old data is outdated.