Shade would appear to be an easy definition, but there are all kinds of shade, when it comes gardening.
Light shade is kind of a moving target. Even if your plants get full direct sun, if the sun is hitting them either early in the morning or late in the day, and lasts only for a few hours, this is considered light shade.
Dappled shade is when the sun reaching your plants is filtered. This is usually because of leaves, overhanging your plants and the light that hits them is dappled, meaning it is spotty.
Part shade is also sometimes interchangeable with "partial sun," but usually means that a plant shouldn't be in direct sunlight during the hottest part of the day and needs 3-6 hours of sun either in the morning or late afternoon.
Full shade does not mean no light, it means no direct sun is necessary for the plant to thrive and/or flower.