A full node is when you download the entire block chain using a bitcoin client, and you relay, validate, and secure the data within the block chain. The data is bitcoin transactions and blocks, which is validated across the entire network of users.
A node which implements all of Bitcoin protocol and does not require trusting any external service to validate transactions. It is able to download and validate the entire blockchain . All full nodes implement the same peer-to-peer messaging protocol to exchange transactions and blocks, but that is not a requirement. A full node may receive and validate data using any protocol and from any source. However, the highest security is achieved by being able to communicate as fast as possible with as many nodes as possible.
Malone, J.A (2015). Glossary of Bitcoin Terms and Definitions. United States: Lulu Press, Inc