Fig 1. Vibration and standing waves in a string, The fundamental and first 6 overtones.
Fig 2. Harmonic series based on G.
The fundamental, also called the fundamental frequency, or first harmonic), is the lowest component in a harmonic series, or the basic frequency of a given note. The fundamental is the loudest element in a note and the one by which the pitch of the note is identified.
In the case of a guitar string, for example, the fundamental is the sound generated by the string vibrating in a single loop along its entire length (see Fig 1).
Taking the note G as the fundamental note, the harmonic series, carried to a reasonable height, would be as shown in Fig 2.