The best replacement for relation is relationship.

The best alternatives to relation are rated as following:
★★★★★  relationship
★★★★  function
★  correlation
★  relative
★  centric occlusion
★  relations
★  centric jaw relation
  foreign key constraint
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Speaking about relation, one probably shouldn't forget about relation.

Other search terms related to relation and their ratings are:
★★★★★ relation
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relation
relationship
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