Slur: "A curved line grouping notes together."
Legato: "Bound together (literally, 'tied'). Without interruption between the notes; smoothly connected, whether in one or several bows."
Portato: "Notes played in one bow-stroke which are articulated without lifting the bow from the string."
Slurred bowings may also be called legato bowings. There is a range of legato characteristics that we use in slurs. We can alter the sustained and smooth nature of legato by adding accents to certain notes or by varying the degree of sustaining with portato bowing.
Resources - Bibliography for Berman, Jackson and Sarch Dictionary of Bowing and Pizzicato Terms
Definitions reprinted with permission from American String Teachers Association.