An image that looks hazy and lacks crisp detail, but is not simply out of focus. Softness can come from a fault in the lens. Often, though, photographers create it on purpose to give a gentle, romantic glow, especially in portraits.
You can make the effect in the camera with a special soft-focus lens, a soft-focus or diffuser filter, or by shooting through thin material, such as a piece of stocking stretched over the lens. It can also be added later with editing software.
Do you have a better definition? Share it in the comments.