Here's a definition from gardening.about.com
Partial Sun / Partial Shade: These 2 terms are often used interchangeably to mean 3 - 6 hours of sun each day, preferably in the morning and early afternoon.
- However if a plant is listed as Partial Sun, greater emphasis is put on its receiving the minimal sun requirements.
- If a plant is listed as Partial Shade, the plant will need some relief from the intense late afternoon sun, either from shade provided by a nearby tree or planting it on the east side of a building.