The following sentences are taken out from a voice-over: The film featured the song Born To Be Wild which contained the lyric "heavy metal thunder". From that point on, thundering rock and roll was referred to as "heavy metal". I don't understand why the singular noun "lyric" is used. My dictionary says "lyric" means a poem. When we want to refer to the words of a song, we say "lyrics".
For your information, the whole voice-over is given below:
Psychedelic rock flourished during the halcyon days of the psychedelic 60s. Its counterculture sound was a perfect fit to America's cultural revolution. In 1968 the hottest counterculture movie was Easy Rider. The film featured the song
Born To Be Wild which contained the lyric "heavy metal thunder". From that point on, thundering rock and roll was referred to as "heavy metal".
If you have time, you can listen to it on the internet:
YouTube - Steppenwolf-Born To Be Wild