Only pure sinusoidal oscillations consist of a single frequency, the fundamental, when considered in terms of frequency, as shown by the Fourier transform.
Distorted sine signals, square wave signals, pulses or sawtooth signals and all other signals are composed of the fundamental and a more or less large number of harmonics. The fundamental of an oscillation and its harmonics are called "harmonics". The fundamental is the 1st harmonic, the 1st harmonic is the 2nd harmonic, the 2nd harmonic is the 3rd harmonic, and so on.
Subharmonics are below the fundamental in frequency, their frequencies are 1/2, 1/3, 1/4, etc. of the fundamental. A frequency component with half the frequency of the fundamental is the second subharmonic.