Distributed consensus is covered in full in the entry Consensus, which explains what agreement means in a blockchain, how networks reach it and how different approaches compare. This page gives a short definition and explains the longer name.
Distributed consensus is the problem, and the set of solutions, of getting many independent computers to agree on one version of shared data even though messages can be delayed, some machines can fail and some participants may lie. In a blockchain, nodes must agree on which transactions happened and in what order, so that the same coins cannot be spent twice. Computer scientists studied the problem for decades before Bitcoin, framing its hardest form as the Byzantine generals problem; systems that solve it are called Byzantine fault tolerant. Practical methods, each described under consensus mechanism, include proof of work, proof of stake and classic voting-based protocols used in permissioned networks.
"Distributed" simply stresses that the agreement is reached among many separate machines rather than set by one central server. In blockchain writing, "consensus" alone almost always carries that meaning, so this site keeps one main entry under the shorter name.