What are the notches on the nickel and on other coins?

Answer 1

The technical name for "notches" on the edge of a coin is reeding, but neither US nor Canadian nickels have reeded edges.

When coins contained precious metals like silver and gold, they were reeded to prevent people from shaving them. Before reeding was introduced crooks would use a knife or file to remove a small amount of metal from the edge, where it wasn't easily noticed. By shaving a large number of coins they could make a reasonable profit from reselling the metal. The addition of reeding made it obvious when coins were being shaved and were a flag to law-enforcement authorities.

Even though modern coins aren't made of precious metals many continue to be reeded, either out of tradition or to help people with limited vision identify coins by touch. For example, European Coins use narrow reeds, wide reeds, and partial reeds to make similar-size coins readily distinguishable.