Rami Arieli: "The Laser Adventure" Chapter 4 Section 1, page 4

 Basic Longitudinal Mode

The last equation of the frequency of longitudinal modes is:

The mathematical expression in parenthesis is the first mode of oscillation available for this optical cavity:
This mode is called basic longitudinal mode, and it have the basic frequency of the optical cavity.

Conclusion:

The frequency of each laser mode is equal to integer (mode number m) times the frequency of the basic longitudinal mode.

From this conclusion it is immediately seen that the difference between frequencies of adjacent modes (mode spacing) is equal to the basic frequency of the cavity:

(Delta n) = c/(2nL)