the degree to which something is poisonous
"The toxicity of the chemical made it fatal even in very small doses."
grave harmfulness or deadliness
"The accidental ingestion of the poisonous mushroom caused severe toxicity that nearly proved fatal within hours."
The quality or degree of being toxic.
"The toxicity of the chemical waste required careful handling before disposal."
In plain English: Toxicity is how poisonous something is and whether it can harm you if you get too much of it.
"The toxicity of these chemicals made them dangerous to handle without protective gear."
The word toxicity comes from combining the root toxic with the suffix -ity. It entered English to describe a state of being poisonous or harmful.