Merkle-Patricia-trie

Merkle tree are Hashes of Hashes, they have the advantage that you can verify only a subtree. Ethereum uses Modified Merkle Patricia Tries for transactions (as well as for state and transaction receipts).

what about Patricia ? What does a trie mean? And how is it used in Ethereum?

Trie (also called digital tree, prefix trie or radix trie)

An ordered tree data structure that is used to store a dynamic set or associative array where the keys are usually strings. A node's position in the tree defines the key with which it is associated.

Merkle Patricia Trie

As described here https://easythereentropy.wordpress.com/2014/06/04/understanding-the-ethereum-trie/, the term Merkle implies that

the root node becomes a cryptographic fingerprint of the entire data structure