ret·ro·ac·tion 
(rĕt′rō-ăkshən)
(rĕt′rō-ăkshən)n.
1. Application of a subsequent rule or condition to something that occurred at a previous time.
2. An opposing or reciprocal action; a reaction.
retro·act′ v.
- sphenoid bone
- spher-
- spher-
- spheral
- sphere
- sphere of influence
- spheric
- spherical
- spherical aberration
- spherical angle
- spherical coordinate
- spherical excess
- spherical geometry
- spherically
- sphericalness
- spherical polygon
- spherical triangle
- spherical trigonometry
- sphericity
- spherics
- spherics
- spherics
- sphero-
- spheroid
- spheroidal