What is a plant?

This itself. It is one of the reasons why truly useful, comprehensive diagnostic keys are so difficult to create; the plant ID key alone would be huge. In diagnosis and treatment, determining whether a plant is a pine or spruce, determining if it is naturally variegated or deciding if it is supposed to be a dwarf are all crucial.
Be cautious with common names. White ash (Fraxinus Americana) and mountain ash (Sorbus americana) are good examples. Both have compound leaves; however, the arrangement is not related. Ash trees have compound leaves attached opposite to one another on the stem while the leaves of mountain ash are attached in an alternate pattern. How the common names are written also show that these trees are unrelated.  Mountain ash is not a “true ash,” so the name is written as a contraction. A hyphen may also be used to denote the same thing as in the case of stinking-ash (Ptelea trifoliata), which is not a “true ash.” Spend time focusing on what plant you are looking at or having described to you. Many diagnoses flounder by initial misidentification. consideration of questions such as the ones that follow.