The second new moon in a single calendar month is called a Black Moon.
While not an official astronomical term, "Black Moon" is commonly used to refer to a few different lunar events, with the second new moon in a month being one of the most prevalent definitions. This occurs because the lunar cycle (the time it takes for the moon to go through all its phases) is slightly shorter than the average length of a calendar month.
It's important to note that the term "Black Moon" is not as universally recognized or consistently defined as, for instance, a "Blue Moon" (the second full moon in a month). There are other interpretations, such as the third new moon in a season with four new moons or a month without a full moon. However, the definition as the second new moon in a single calendar month is widely recognized.
