1,238. Defining Some Terms

Hilchos Kiddush HaChodesh 1:3

The moon is concealed and cannot be seen for two days more or less each month. This occurs about a day before its conjunction with the sun at the end of the month and the day after its conjunction, after which it can be seen in the west in the evening sky. The first night when the moon can be seen in the west after not being visible is the start of the month, days being counted from that day. If the moon is observed on the night of the 30th day, then the 30th day will be Rosh Chodesh. If the moon is not observed, Rosh Chodesh will be on the 31st day and the 30th day will be the last day of the previous month. It is not necessary to observe the moon on the 31st night; it is Rosh Chodesh regardless of whether or not the moon is seen. This is because no lunar month ever exceeds 30 days.

Hilchos Kiddush HaChodesh 1:4

When there are 29 days in a month because the moon was observed on the 30th night, the month is called lacking (“chaseir”). If the moon is not seen so that the preceding month has 30 days, that month is called full (“meubar” or “malei”). If the moon is seen on the 30th night, it is said to have been observed “at the proper time”; if it is only seen on the 31st night, it is said to have been seen “on the night of its fullness.”