World lock was the term used for a world that couldn't be accessed from other worlds, or only from a select few worlds. Same went the other way around - it was impossible to get out of a locked world. A world lock ccould be artificially created by magic users , but it demanded a lot of energy. More common was natural locks. Both artificial and natural locks could be disturbed during outbursts of magical disorder.
A famous example of a locked world was world 12.