O
obesity
having a body mass index (BMI) of 30 kg/m2 or greater; above-normal body weight
offspring [off spring]
The outcome of successful breeding of an animal or plant.
oncogenes
genes that cause the cells to transform or become cancerous
organism [or gan is em]
Any living thing, either plant, animal or microbe.
osteoporosis [os tee u por o sis]
a condition characterized by a decrease in bone mass and density, causing bones to become fragile and easily broken.
oxidant [ox si dunt]
A substance that takes electrons away from another substance during a chemical reaction that makes a new chemical bond.
oxidation [ox i day shun]
When oxygen is combined with another substance. This causes a chemical reaction that damages cells.
ozone [oh zon]
The oxygen form that has 3 molecules linked together with a formula O3. At the upper layer of the atmosphere ozone forms a layer that protects and screens some UV radiation.
ozone layer [oh zone • lay er]
A layer in the earth's atmosphere from 10 to 30 miles high, where most ozone collects. This layer absorbs all of the form of radiation called ultraviolet-C radiation. It also absorbs most of the radiation called ultraviolet B. Often this radiation is simply called UVB.