n (chemistry) An allotrope of oxygen (symbol O3) having three atoms in the molecule instead of the usual two; it is a blue gas, generated from oxygen by electrical discharge; it is poisonous and highly reactive, but in the upper atmosphere it protects life on Earth from ultraviolet radiation.
n (context|from an erroneous former belief that seaweed contains and releases ozone) fresh | Fresh air, especially that breathed at the seaside and smelling of seaweed.