Merged mining is a process of mining two or more cryptocurrencies at the same time, without sacrificing overall mining performance.
History of the concept
Discussion at the bitcointalk forum
Merge mining was first described at the BitcoinTalk forum in 2010. This idea got a positive feedback from the anonymous creator of Bitcoin:
"Instead of fragmentation, networks share and augment each other's total CPU power. This would solve the problem that if there are multiple networks, they are a danger to each other if the available CPU power gangs up on one. Instead, all networks in the world would share combined CPU power, increasing the total strength. It would make it easier for small networks to get started by tapping into a ready base of miners."
Merged mining was first implemented in Namecoin in 2011 and then was used in many other projects.
Merged mining of altcoins
People are familiar with this term thanks to altcoins such as Namecoin, which is merge-mined with Bitcoin, and Dogecoin, which is merge-mined with Litecoin.