Easily-confused!
If your specimen is a:and has stem­/­leaf ar­range­ment:and the leaves have:and the bark is/has:and the twigs:and the fruit (if present):then you have:
tree╫ opposite (most species) ___ smooth,
taper at both ends, and
pass the dogwood leaf test
no con­spic­u­ous lenticels nor strong colors lookalike dogwood genus Cornus
sub-opposite small ▲▲ teeth dark, lots of light-colored lenticels ;
inner bark=­or­ange (scrape with knife)
often have a terminal thorn are small, black, taste dis­gust­ing after a few seconds—spit it out lookalike common buckthorn Rhamnus cathartica
╟ alternate ___ smooth or
very small ▲▲ teeth (use 🔎︎ hand-lens)
dark, lots of light-colored lenticels no terminal thorn ;
hairy
lookalike glossy buckthorn Frangula alnus
▲▲ teeth, visible within arm's length dark, lots of light-colored lenticels ;
if mature, looks covered with burnt potato chips­/­crisps
are small, black, taste like 🍒︎ cherry even if sour lookalike 🍒︎ black cherry Prunus serotina
no more than waist-high (<1 m) (<3 ft)▲▲ teeth no hair this plant, please continue below . . .

Verify with Google images.

Also called alder buckthorn (although do not confuse with glossy buckthorn Frangula alnus which is also called that).

Native to North America.

Uses by native peoples
(Ethnobotany database)

Learn more about alderleaf buckthorn Rhamnus alnifolia

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