The prefix 'auto' means self
- automatic means self moving
- an autobiography is a self-written biography.
- an automobile moves without humans pushing it along.
Plants are autotrophs.
- They are able to make their own food.
- So, autotroph means automatic nourishment.
- Photo- means light related, so putting it all together photoautotrophic means light related automatic nourishment.
A better definition is 'the use of light to synthesise - or make - organic molecules'.