Abstract / Concrete
Hegel (pervasive)
Not "abstract = theoretical" vs. "concrete = practical." For Hegel: abstract = one-sided, isolated, poor in determinations. Concrete = mediated, rich in determinations, comprehended in its interconnections. The concrete is the "grown-together" (lat. *concrescere*).
Origin
An inversion of ordinary usage. "Being" sounds concrete but is abstract (empty). "Concept" sounds abstract but is concrete (richly determined).