A slur is a symbol in Western musical notation indicating that the notes it embraces are to be played without separation, i.e. with legato articulation. A slur is denoted with a curved line generally placed over the notes if the stems point downward, and under them if the stems point upwards:

The slur is not to be confused with two other similar musical symbols. The tie is a curved line that links two notes of the same pitch to show that their durations are to be added together. The phrase mark is a curved line that extends over a passage which is visually indistinguishable from the slur, and indicates that the passage is to be interpreted as a single phrase.