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oxygen a gas with no color or smell that is one of the chemical elements. Oxygen combines with hydrogen to make water. It also makes up about twenty percent of the earth's atmosphere. Most living things need oxygen. (symbol: O)
oyster a small animal with a soft body and a hard shell in two pieces. Oysters have a rough shell with a very shiny inside, where pearls sometimes grow. Oysters live in shallow ocean water. They are mollusks and are closely related to clams and mussels. People eat some kinds of oysters.
oz. an abbreviation for ounce or ounces.
ozone a form of oxygen that occurs when oxygen is exposed to an electrical charge, like lightning. It is found naturally in Earth's atmosphere.
ozone hole an area in the ozone layer where the ozone becomes thin. There is a large ozone hole over Antarctica and a smaller one over the North Pole. Scientists warn us this is dangerous.
ozone layer an atmospheric layer between ten and twenty miles above the earth that contains a large amount of ozone, which absorbs certain types of harmful radiation from space.
P2 symbol of the chemical element phosphorus.
p the sixteenth letter of the English alphabet. [2 definitions]
p. page.
PA1 an abbreviation for Pennsylvania.
pace1 the length of one step; stride. [8 definitions]
pacemaker a small electronic device put under the skin to control the heart rate in people with heart problems.
Pacific the largest ocean in the world. It lies between North America and Asia in the north, and South America and Australia in the south; Pacific Ocean.
Pacific Ocean the largest ocean in the world, separating Asia and North America, and Australia and South America, and extending to Antarctic; Pacific.
pacify to calm or bring back peace of mind to.
-pack a suffix that means a package containing that number of items.
pack1 a group of things arranged together in a container; package. [8 definitions]
package an object or bundle that is packed, wrapped, or boxed; bundle. [3 definitions]
packet a small bundle or parcel.
pact an agreement or a sworn promise. [2 definitions]
pad1 a piece of soft material used as cushioning for protection or comfort. [7 definitions]