an exercise device consisting of an endless belt on which a person can walk or jog without changing place
"She used the treadmill to maintain her fitness routine while staying in the living room during the storm."
a mill that is powered by men or animals walking on a circular belt or climbing steps
"The ancient factory employed several treadmills where oxen walked in circles to drive the grinding stones."
A piece of indoor sporting equipment used to allow for the motions of running or walking while staying in one place.
"She stepped onto the treadmill to run indoors without leaving her apartment."
In plain English: A treadmill is an exercise machine that lets you walk, jog, or run while staying still by moving a belt underneath your feet.
"She walked on the treadmill every morning to stay fit before work."
Usage: The term "treadmill" refers specifically to the machine itself, whereas "to treadmill" is an informal verb meaning to run continuously without progress; avoid using the noun form as a metaphor for futile effort unless emphasizing the mechanical nature of the activity.
To exercise on a treadmill.
"I decided to go for a run on the treadmill after the rain started pouring outside."
In plain English: To treadmill means to keep moving forward without actually making any real progress toward your goals.
"You never stop working at that job, so you seem to be just treading water instead of moving forward."
The word combines tread with mill, referring originally to machines used for grinding grain or walking exercise equipment. It now also describes any situation where one must keep working hard just to stay in the same position.