A tree is a woody plant with one main, erect trunk with a circumference of at least 9 1/2 inches (or a diameter of 3 inches) at breast level (which is considered 4 1/2 feet above ground level) at maturity and a rather distinct and elevated head (crown). If not altered through human intervention, true trees (e.g., American elm trees) will, by definition, generally reach a height of 13 feet or more.
Note that any "tree" definition must admit of exceptions where, for example, a plant that is clearly a tree has been injured and, due to the resulting damage, fails to conform to the definition. We know red oaks to be trees, and we are not swayed to renounce this conviction by an instance where a red oak tree has been damaged and assumes a multi-trunked (i.e., shrub-like) growing habit.
Also note that a plant classified as a shrub can, in some cases, develop trunks with a circumference of at least 9 1/2 inches at breast level. But the key distinction here is between singular and plural. A tree unaltered by humans or by injury will, by definition, have one main trunk (singular) that measures at least 9 1/2 inches around at breast level. I recently measured some old PeeGee hydrangeas in a cemetery and found that, at breast level, they had multiple trunks with a circumference of 9 1/2 inches or more. This multiplicity disqualified them from being trees, technically speaking.
More Examples of Trees:
