The term "old growth" is one often used and perhaps less-often understood.
Just big trees? Big trees and spotted owls?
No and not quite are the answers; Joan Maloof provides a bunch more in her book Nature's Temples.
Maloof is the director of the Old-Growth Forest Network, dedicated to preserving forests undisturbed by humans from coast to coast. She helps us understand all the many forces at play in a thriving old-growth ecosystem.