usually clustering refers to having a series of servers that all work for the same site. This keeps a high traffic website running smoothly because when 1 server gets loaded another takes over. This also works as a fail over meaning that if one server goes down your site still has other servers to depend upon so your site does not go down.
Connecting two or more computers together in such a way that they behave like a single computer is called clustering. Clustering is used for parallel processing, load balancing and fault tolerance.
Hi there, I have never heard this term in SEO, so I think it is not related with SEO. In computer science clustering is a group of 2 or more than 2 computers linked such that they form a single unit in many aspects. But it's not necessary that all the components of different computers are connected with each other.