Hopefully, I've learned something the past few months. I don't have it all down-pat yet, but... From Sibley's "Birding Basics" regarding the Life Year System of molt terminology:
At any time during the first year of life from hatching until the following summer when it is about twelve months old, a bird is called a first year. The first year can be subdivided into juvenile, first winter, and first summer. In its first flying stage, just after the downy plumage worn in the nest, the bird is called a juvenile.
The Humphrey-Parkes system has a slightly different nomenclature. Again from "Birding Basics":
The H-P system labels the plumage in the bird's first flying stage as the juvenal plumage.
I don't see "immature" as a bold-typed official word in the molt terminologies he discusses, but in my limited understanding/exposure, "immature" is a bird who has molted out of Juvenal plumage, but has not molted into adult plumage yet. Some species molt into adult plumage in their second year, others take longer. It's pretty interesting stuff, whether a bird has one or two molts per year ("basic" versus "alternate"), and whether the species has an additional molt in the first year ("simple" versus "complex").
