In Arrow, both Sarah and Laurel are called "Dinah", i.e. "Dinah Laurel Lance". What does it mean?
Laurel's full name on the show is Dinah Laurel Lance. She's named after her mother, Dinah Lance, so to avoid confusion everyone calls her by her middle name, Laurel.
As far as I know, Sara has never been called Dinah on the show.
In the pre-Flashpoint DC comics, Dinah Drake was the original Black Canary. She married detective Larry Lance, and they had a daughter, Dinah Laurel Lance, who would become Black Canary after her mother.
In the Arrowverse, Laurel's mother was never Black Canary, but her sister was. From the beginning, the name was a clue to DC fans that Laurel was destined to become Black Canary at some point, which she did.
(In the New 52 reboot, Laurel does not yet exist; Dinah Drake is/was married to Kurt Lance, and is still Black Canary.)