Definition of the time called "Between the Suns [Bein Hashemashot]": Berachot 2b
Considering this time of day doubtful, as to whether Shabbat has begun or not: Eruvin 36a-b, 76a
Performing prohibited melachah during this time
Penalty for performing melachah during "bein hashemashot", as Shabbat begins/ends, or as Yom Kippur begins/ends, so that only part occurs on Shabbat itself: Keritot 19a, 19b
