The scietific definition of toxin refers to that which is produced by organisms (animals, plants, bacteria, fungi, etc.) which can either be produced naturally or through biogenetic engineering.
However, in common vernacular the term has become used as a catchall phrase for all poisonous substances whether manmade (chemicals, byproducts, etc.) or naturally occuring.
IMO sort of like the use of scientific names vs common names. Toxic vs. toxin - many chemicals can produce toxic reactions, be toxic to the environment, etc. the two words have blended in common speech.
How about this:
Toxin:
1. A chemical compound that is produced by an organism and generally causes harm when introduced into the body tissues of another organism. Certain species of salamanders (e.g., Taricha granulosa, Notophthalmus viridescens, etc.) produce toxic secretions as a defense mechanism that result in the salamander becoming minimally unpalatable or maximally lethal to many predators.
2. A catchall term commonly used to to describe poisonous substances whether naturally produced or manmade. Examples of the latter might include chemicals, byproducts, contaminants, etc., referred to as environmental toxins, reproductive toxins, etc.
