Chain Condition

In genrative syntax, a chain condition is a wellformedness condition on chains, which states that every chain (a1,...,an) must contain exactly one Theta-marked position and exactly one Case-marked position.

Comments
In A-chains an is Case-marked and an is Theta-marked, while in A'-chains the lowest variable A'-bound by the operator is Case-marked, and may either be Theta-marked, or A-bind a Theta-marked position in the chain. This condition restates the case filter (or the visibility condition) and the Theta criterion in terms of chains.

Link
Utrecht Lexicon of Linguistics