What is indicated by the term 'Phon'?

Noise may be expressed in psycho-acoustic terms - the phon. The phon is a psyho-acoustic index of loudness. It takes into consideration intensity and frequency. Also remember: "Octave Band Frequency Analyser," which measures the noise in octave bands. The resulting plot shows the "sound spectrum" and indicates the characteristics of the noise, whether it is mainly high-pitched, low-pitched or of variable pitch. Ref: Park's textbook of Preventive and Social Medicine, 21stedition, page-684