Adaptations are traits which enhance an organism's ability to survive and thrive in their environment i.e. traits which enhance their evolutionary fitness.
For example, the following description of Eucalypt trees from Charles Darwin’s journal describes some of the unique adaptations Eucalypts have developed in response to the Australian conditions.
“The extreme uniformity in the character of the Vegetation, is the most remarkable feature in the landscape of all parts of New S. Wales….The tree’s nearly all belong to one peculiar family; the foliage is scanty and of a rather peculiar light green tint; it is not periodically shed; the surface of the leaves are placed in a vertical, instead of as in Europe a nearly horizontal position…”
Adaptations are generally sorted into three different types: behavioural, structural, and physiological. This is outlined in the following video.