Why do we call sending a child to his/her room as "grounded" ?

It was originally a piloting term that got borrowed by parents. When pilots aren’t allowed to fly for some reason—including bad behavior—they are said to be “grounded” (since they are stuck on the ground). Since this is similar to punishing a child by keeping them in one place and not letting them go where they want, the term got applied in that context as well (originally as slang, but then it stuck).

I never associated the two as being the same.
You can be sent to your room, usually as a temporary punishment, or you can be grounded (unable to leave, say, the house) for a period of days or weeks. To restrict, as to ground a pilot.
To say that one is grounded in his or her belief, for example, might mean their belief is sane, rational, “down to Earth.”

It’s when you attach a green insulated wire to them that’s pathed from the earth.

We don’t. Being sent to one’s room is to be separated from the social and entertainment center of the house. (It was more meaningful when there was only one television in the house.)

Being grounded means one does not get to go anywhere but home, school, and church.

We don’t. Being grounded means staying home and isolated from friends for a period of time.

Sending a child to their room usually only lasts a few hours, whereas grounding is usually measured in days or weeks, or in extreme cases, months.

I used to be grounded for the weekend, or once for a month. I’d be in the room with the radio or cassettes, and maybe I would talk on the phone (phone was in the kitchen, not in my room).

I don’t ground my daughter but if she were made to stay home, it wouldn’t be punishment. She likes staying home. What would be more of a threat is taking away her cell phone and devices.

I explain to her all the time that when I was her age, we had no WiFI, no computers, no phones – it was really like “Little House on the Prairie.” lol

@jca2 no phones, no lights, no motorcars, not a single luxury. Like Robinson Crusoe, we were primitive as can be

So, you have had a number of descriptions here for grounding. Permit me to clarify the other half.
Sending a kid to their room is a time out. The term came along in recent years, but that’s what it is.

For me, grounded just meant having to stay home, not necessarily in my room.