The term concession is from the mid-15c., "act of granting or yielding" (especially in argumentation), from Old French concession (14c.) or directly from Latin concessionem (nominative concessio) "an allowing, conceding," noun of action from past participle stem of concedere "to give way, yield," figuratively "agree, consent, give precedence," from con + cedere "to go, grant, give way."
Negotiations by nature are a debate to see who can get what one is asking. Rarely are do the results of a negotiation match the start. What happens to change the ending? Concessions change the ending of a negotiation. Yield. Grant. Give away. Never split the difference.
Abstract reasoning, create the calculation, solve the mystery