A relationship between two variables where they increase or decrease at the same rate, maintaining a constant ratio. When one variable doubles, the other doubles; when one triples, the other triples, creating a linear relationship that passes through the origin.
From Latin 'directus' meaning 'straight' and 'proportio' meaning 'comparative relation'. The concept was formalized by ancient Greek mathematicians and later developed by medieval Islamic scholars who studied proportional relationships in geometry and algebra.
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