any of numerous varieties of small to medium-sized round or oval fruit having a smooth skin and a single pit
"The gardener picked several ripe plums from the tree after they turned deep purple."
The fruit and its tree.
"The gardener pruned the plum tree to encourage more fruit production next summer."
The edible, fleshy stone fruit of Prunus domestica, often of a dark red or purple colour.
"The gardener picked a ripe plum from the tree to eat its sweet flesh."
In plain English: A plum is a sweet, juicy fruit with smooth purple skin and soft flesh inside.
"She added fresh plums to her summer salad for a burst of sweet flavor."
Usage: Use the noun form to refer specifically to the sweet, juicy fruit with a hard pit inside. When describing color as an adjective, apply it only to shades resembling that ripe fruit's deep reddish-purple hue.
To plumb.
"The engineer used sonar to plumb the depth of the ocean trench."
Of a dark bluish-red colour.
"The new skyscraper rises straight up to be perfectly plumb against the horizon."
Plumb
In plain English: When something is plum, it means it is completely full to the brim with no empty space left.
"The plum job comes with a high salary and great benefits."
A surname.
"Mr. Plum was elected mayor last month."
The word plum comes from the Latin prūnum via Old English and Middle English. It originally referred to the fruit known today as a plum.