Uhm ..... for me, it's a square or rectangular reflecting surface, that was originally round .....
(just kidding )
Dish ? ..... ashtray ? ..... cover plate ? ..... cigarette holder ? ..... lighter ? ..... polished steel cover ? ..... do you realize that with the premises you gave, it can be literally everything ?
1. It is square or rectangle when delivered to the consumer.
2. This thing comes from something that is round at first. Maybe about 15 inches across.
3. There are two main manufacturers of these items plus some smaller companies that make them.
And here's number four:
4. It will fit in the palm of you hand.
My only guess is a silicon wafer. It's circular when delivered to a buyer, but gets cut into squares to be sold to consumers (in processors). I suspect your two major manufacturers are Intel and AMD.