The Rosenthal effect is another name for the Pygmalion effect. It describes how a teacher, leader, or coach can improve the performance of those they are leading by consistently having, and expressing, high expectations of them.
It is named after one of the researchers (Lenore Jacobson and Robert Rosenthal) who first described the effect. In short, it shows how low expectations of others can lead them to perform badly, while high expectations can lead to higher performance.