Metallic disc with a very small hole in its centre, with which the objective of photographic cameras can be substitute in some occasions.
This is the way the dictionary defines the term pinhole, one of the main parts in the pinhole cameras. But it is the hole itself made on the metal what is related with the pinhole.
A small 0.50 mm. thick brass plate is enough. It is very important that the pinhole is perfectly round and has no rough edges at all. If so, the image in the inside could be deformed, with stains or shadows.
The best solution for getting this perfect hole will be an adequate drill bit to the diameter we need.