Definition - What does Pinhole mean?
A pinhole is the occurrence of a leak in a metallic (usually copper) pipe. These leaks are approximately the size of a pinhole.
Pinhole leaks form as a result of pitting corrosion on the pipe's internal surface, resulting in pressure being exerted by the flowing water, which over time erodes the copper material.
Corrosionpedia explains Pinhole
Pinhole leaks are primarily observed in small diameter piping systems that carry water for domestic or industrial purposes. These small diameter pipes were made from copper with an assumption that the pipes would last for 20-25 years. However, some copper pipes only last for two years due to the corrosive nature of copper when in contact with water and oxygen.