Cusp
The everyday word for the day a sign changes over — the boundary itself, not a blend of the two signs on either side of it.
People born on a sign's changeover day often call themselves 'born on the cusp,' as if the day carried a bit of both signs at once. It doesn't, really: the Sun crosses into the new sign at one exact instant, and everything before that moment belongs fully to the old sign, everything after fully to the new one. What reads as a blend is usually just uncertainty about which side of that instant a birth actually fell on. A cusp birthday is resolvable, not ambiguous, once the exact birth time is checked against the ingress instant for that year — which is the whole reason this site gives ingress times to the minute rather than leaving the day to stand in for the moment.