Antonym of "distributed" as in "distributed system"

If we consider context of computing, what is the opposite of distributed? Is it single or combined ? What is the opposite of "distributed system"?

In the context of databases and software applications in general, the antonym of distributed is centralized.

Probably depends on the precise situation, but for written technical use I would go with "co-located". "Distributed" implies different things in different locations, where "co-located" implies different things in the same location. Both of your suggestions actually suggest only a single thing (in one way or another)

Or perhaps an appropriate word is "localized."

Concentrated is an antonym for 'distributed' from its connotation of 'spread out'.

Some distributed computing frameworks [1] also use the term single node setup. This would match a context where distributed mainly refers to the fact that computation runs across multiple compute nodes. In this context, something like local computation might be appropriate as well.

[1] https://community.hortonworks.com/questions/203614/-what-is-cluster-single-node-cluster-and-node.html