the.com/tree ring

means One of the concentric circles visible in a cut tree trunk, each marking a year of growth, with its width recording how good or hard that year was.

from Straightforward English compound — 'tree' plus 'ring,' describing exactly what you see: a ring-shaped layer in the wood. The science behind it, dendrochronology (from Greek 'dendron,' tree, plus 'khronos,' time, plus '-logia,' study), only got its formal name in the early 20th century, once researchers realized these rings could be counted and cross-matched to date timber, droughts, and even volcanic winters. The rings themselves are simpler than the science: each year a tree grows a pale, fast band in spring and a darker, slower band in late summer, and the two together make one ring — one year, visibly banked.