tree or other plant that does not shed all of its foliage annually. but rather progressively over a period of years (contrast with deciduous).
Merriam-Webster Online Dictionary
evergreen (adjective)1.
having foliage that remains green and functional through more than one growing season - compare deciduous 1
2.
retaining freshness or interest - perennial
1.
an evergreen plant , also - conifer
2.
twigs and branches of evergreen plants used for decoration
3.
something that retains its freshness, interest, or popularity
evergreen (Wikipedia)
A silver fir shoot showing three successive years of retained leaves.
In botany, an evergreen is a plant that has leaves throughout the year that are always green. This is true even if the plant retains its foliage only in warm climates, and contrasts with deciduous plants, which completely lose their foliage during the winter or dry season.