When a melodic idea is presented by one voice or instrument and then restated immediately another what the technique called?

The technique is called imitation. It is a compositional device where a musical phrase or idea is repeated in a different voice or instrument, usually immediately following the original statement. Imitation can be used to create a sense of unity and coherence in a musical composition, as well as to add variety and interest. It is often used in canons and fugues, but can also be found in other genres of music.