What are 'factors' in biology?
There are many meanings of "factor" in Biology. In the context where you place it, namely bio-active substances, such as hormones and enzymes I would go for the definition given by the Medical Dictionary:


  factor [fak´ter] :
  an agent or element that contributes to the production of a result.


Within this definition, a transcription factor contributes to the transcription of DNA into mRNA, and nerve growth factor contributes to the growth and survival of neurons.