Some mutations have no observable effect on the phenotype. what is the term for this type of mutation?

The answer to this question would be silent mutation.

Silent mutation is a mutation in the DNA that was not significant so that the phenotype of the organism is not changed. The mutation might change one of the amino acids sequences in the DNA but have the new protein made from the mutation have a similar function like the normal DNA. Silent mutation will not have any effect to the organism but might be dangerous if accumulated.