Arbitrate is the verb: to make a judgment on a dispute as a neutral third party. Arbiter is the noun: the person, or sometimes the thing, doing that judging. If you need an action, use arbitrate; if you need to name the judge, use arbiter.
The core difference
Arbitrate is the verb for settling a dispute; arbiter is the noun for the person who settles it.
- arbitrate — to make a judgment on a dispute, or submit a dispute to such judgment: the union and the company agreed to arbitrate the pay dispute.
- arbiter — someone chosen by two sides to settle a dispute between them, or something with the power to judge or decide: a retired judge was brought in as arbiter to settle the boundary disagreement.
How to tell them apart
Arbitrate and arbiter both trace back to the Latin arbiter, meaning a witness or judge, so it is no surprise they are close cousins in meaning. The difference is simply one of grammar: arbitrate is what an arbiter does. An arbiter arbitrates; arbitration is the process.
A quick check is to try swapping in a role noun. "She acted as arbiter" works because arbiter names a role, like referee or judge. "She acted as arbitrate" does not work, because arbitrate is an action, not a role. Arbiter can also stretch beyond people, to anything with deciding power, as in critics being called the arbiters of good taste.
