ETYM Old Eng. resoun, French raison, from Latin ratio (akin to Goth. rathjô number, account, garathjan to count, German rede speech, reden to speak), from reri, ratus, to reckon, believe, think. Related to Arraign, Rate, Ratio, Ration.
1. A fact that logically justifies some premise or conclusion.
2. A rational motive for a belief or action; SYN. ground.
3. An explanation of the cause of some phenomenon.
4. The capacity for rational thought or inference or discrimination; SYN. understanding, intellect.

1. To decide by reasoning; draw or come to a conclusion; SYN. reason out, conclude.
2. To think logically.