What does the circle with a line through it mean?

[pukupi]

If it had a cross instead of a single line, it would mark the centre of gravity but I've often wondered what is the significance of circle with a line through it on just about every camera I have?

 

[RML]

It marks the film plane.

 

[pukupi]

Thank you. This explains its unusual location on my dSLR.

 

[Pherdinand]

A very handy tool when you want to measure the distance accurately that you'll set on the lens for focus.

 

[peter_n]

Indeed. How come there isn't one on my M6? 🙁 My Nikon F3 has one...

 

[rbiemer]

It also means "diameter", depending where the mark is. If it's on the top deck of a camera and the line is parallel to the camera back--that's the film plane. if it's on a lens and followed or preceded by a number then it's usually the filter diameter.
Rob