Thanks Judy - this is a great topic. I can't define softness or lightness for myself without first defining contact. A lack of contact can be neither soft nor light - although I will often ride on a long rein deliberately. I like to feel an elastic butterfly connection to my horse - although I have been criticized for sometimes having insufficient contact. - So I guess elasticity has to be a component here. Sometimes if my horse needs support I will increase the contact - the pressure on the reins in the rein/body/leg circle - but this requires adjusting the rest so the horse knows stop right away or shorten your frame etc. This can still be soft in my mind, but is maybe no longer as light.
But really, I think these are just adjectives to describe a contact rather than definitions - these must mean different things to all of us - especially considering our different riding styles. Feel free to criticize - this is only an idea and I welcome feedback. Laura.