In horticulture heavy shade is a planting location that sees hardly any light during the day, and to expect a hedge to grow in heavy shade is hopeful.
For an example, along with a few other species, a holly hedge will survive being planted in a lot of shade, but they will not produce as much leaf and will therefore grow leggy stems and branches that reach out for the light. They can become unruly, gappy and see through.
Partial shade is a different matter and there are many species of hedge that will happily grow in partial shaded locations.
See our hedges section for more detailed information.