ratio (n.)

1630s, "reason, rationale," from Latin ratio "reckoning, numbering, calculation; business affair, procedure," also "reason, reasoning, judgment, understanding," from rat-, past participle stem of reri "to reckon, calculate," also "think" (from PIE root *re- "to reason, count"). Mathematical sense "relation between two similar magnitudes in respect to quantity," measured by the number of times one contains the other is attested from 1650s.