This is the Wikipedia definition of a harmonic:
A harmonic of a wave is a component frequency of the signal that is an integer multiple of the fundamental frequency, i.e. if the fundamental frequency is f, the harmonics have frequencies 2f, 3f, 4f, . . . etc.
So, the second harmonic is 2 times the fundamental.
But 1 is also an integer and it is an odd number. So, the fundamental is the 1st harmonic and it is an odd harmonic.
This is from Wikipedia's article on overtones:
Frequency... Order ...Name 1 ...Name 2 ...Name 3
1 · f = 440 Hz...n = 1...fundamental tone 1st harmonic ...1st partial
2 · f = 880 Hz...n = 2 ...1st overtone...2nd harmonic ...2nd partial
3 · f = 1320 Hz...n = 3 ...2nd overtone ...3rd harmonic ...3rd partial
4 · f = 1760 Hz ...n = 4 ...3rd overtone ...4th harmonic...4th partial