Consensus - a general agreement about something : an idea or opinion that is shared by all the people in a group.
Term Decentralised Consensus used in connection to technology when transactions are recorded into a distributed public database. It is used in a number of ways.
For example in the case of Bitcoin consensus achieved by a proof-of-work system 
called Mining.
Disclaimer: This is a confusin term, often used in a wrong way due to the fact that it came into fashion. Here I will try to explore it in a way that it makes more sense and popularise theterm with the minimum amount of BS.
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