The description of the word you requested from the astronomical dictionary is given below.
A month is a period of time that is (historically) tied to the motion of the Moon around the Earth. Quite a few different kinds of months are in use:
De next table shows the average length of the astronomically determined months at 1 January 2000, and by how much this length changes in a century (derived from Meeus: Astronomical Algorithms, ISBN 0943396352). For example, on 1 January 2000 the average length of the synodical month was equal to 29 days, 12 hours, 44 minutes and 2.877 seconds, and this length increases at 0.019 seconds per century.
Table 4: Month Lengths