Thicket is a densely packed forest area. At this time there have been reports of the plants growing within Thicket to be similar to the plants of Grassland or Broadleaf Forest; however, if plowed it reverts to fir forest land. Thickets appear as white on the minimap.
Walking within the thicket at anything more than a crawl uses stamina very quickly. Plowing the thicket also drains stamina very quickly, even if it's the patches that look like normal terrain.
An example of thicket terrain.