Final answer:
The term that matches the synonyms harmful, poisonous, and lethal is toxic.
Step-by-step explanation:
The term that aligns seamlessly with the synonyms harmful, poisonous, and lethal is "toxic." This adjective encapsulates the essence of substances or elements that pose a threat to living organisms, conveying the potential for harm, often in a deleterious or fatal manner. Whether applied to chemicals, relationships, or environments, "toxic" implies a detrimental impact that can compromise well-being. In the realm of environmental science, it denotes substances capable of causing ecological harm. This versatile term serves as a comprehensive descriptor, succinctly conveying the inherent danger associated with various contexts, and its usage spans from toxic chemicals in industry to toxic behaviors in interpersonal dynamics, fostering clarity in communication.