Slur (music)

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:

A slur can be extended over many notes, sometimes encompassing several bars. In extreme cases, composers are known to write slurs which are near-impossible to achieve; in that case the composer wishes to emphasise that the notes should be performed with as much legato as possible.

...
Wikipedia