2,071. Days of Niddah vs. Days of Zivah

Hilchos Issurei Biah 6:4

All seven days starting with the day on which a woman is expected to menstruate are called “the days of niddah”; this is the case whether she actually menstruates or not. They’re called the days of niddah because they are days she could become a niddah, and any blood found in this time is considered niddah blood.

Hilchos Issurei Biah 6:5

The eleven days following these seven are called “the days of zivah.” Again, this is the case regardless of whether the woman bleeds at this time or not. These are called the days of zivah because a woman could experience zivah at this time and any blood found on these days is considered zivah blood. One should make note of the terms “days of niddah” and “days of zivah.”