describes an orientation
In the inverted position. With the bottom at the top and the top at the bottom.
If something is upside down, then you will need to rotate it through one hundred and eighty degrees to put it the right way up. Upside down defines the orientation of an object and its opposite is the right way up. There is an upside down banana cake, where the cake is on the top and the filling is at the bottom The normal order of layers has been inverted.
For example: You'll have to turn the ketchup bottle, if you want to get the ketchup out.
The letter S looks the same upside down and the right way up.
If you hang upside down for too long all the blood rushes to your head.
Very often I write an address on an envelope only to discover that I have written the address upside down in relation to the flap.
I packed the shampoo upside down in my suitcase and it has leaked everywhere. (adverb)
We hung the picture upside down. (adverb)
We often use upside down in a figurative way to mean disorder or mess.
For example: The office had been turned upside down, there were papers and broken glass everywhere.
The divorce turned my life upside down.
To turn upside down can be used to mean to look everywhere, to search thoroughly. Very often to turn inside out is used with the same meaning.
For example: The police turned the town upside down looking for the missing child.
I turned my room upside down looking for my keys.
If you have something upside down, it could be used to mean that you are confused or have misunderstood something.
For example: You have got my instructions completely upside down.