I'd say parental input is enviromental, but it is much more intensive and interactive than that of other environmental input, so it should be seperate. A parent communicates with their child far more frequently than anyone/anything else in the child's environment.
Environmental input will include general language stimulation, such as hearing conversation and watching television. Despite the general consensus that 'watching tv is bad for your kids', they will actually learn quite a lot from Children's TV programmes. As long as there's a balance between all forms of interaction and communication, there's no problem.
Also, I'd like to say 'parents/carers', since it's not always the parents who care for children.