P.o. is the abbreviation for per os, signifying oral administration (e.g., that a drug is to be swallowed).

2. P53 gene
Definition:

A gene which encodes a protein that regulates cell growth and is able to cause potentially cancerous cells to destroy themselves. The gene is thus an antioncogene, and is often found mutated in smokers with lung cancer by a compound found in tobacco smoke, a polycyclic aromatic hydrocarbon called benzo[a]pyrene. The protein it encodes is a phosphoprotein that is 53 kilodaltons in mass.

3. pacemaker
Author: Guo Li
Definition:

A pacemaker is something which initiates an action and controls the rhythm of a series of actions. The physiologic pacemaker of the heart is the sinus node. An artificial pacemaker is a device implanted in a patient which functions when the physiologic pacemaker fails.

4. packed cells
Author: Guo Li
Definition:

This refers to the concentrated red blood cells used for transfusion. Each unit of whole blood (about 450 ml) obtained from a donor is mixed with preservatives containing: citrate, phosphate, dextrose and adenine. It is centrifuged to separate the plasma (about 200 ml) from the red blood cells (about 250 ml).

5. paclitaxel (Taxol)
Image: View the chemical structure
Definition:

This alkaloid is an extract from the Pacific yew, Taxus brevifolia. It inhibits microtubule disassembly and is used as a chemotherapy agent to treat bladder, esophageal and lung cancers and is being investigated as a treatment for leukemia and lymphoma.

6. Paget's disease of bone (osteitis deformans)
Author: Guo Li
Definition:

Early symptoms of Paget's disease include softening of bones and bending of weight-bearing bones. Late symptoms include thickening of bones. This is essentially a disease of the elderly as the median age at presentation for treatment is about 70 years.

7. Paget's disease of the breast
Author: Guo Li
Definition:

This is a chronic eczematous condition or ulceration of the nipple which is associated with infiltrating ductal carcinoma.

8. pain
Author: Guo Li
Definition:

Pain is a sensation of suffering due to some form of stimulation of nerve endings. Pain is characterized by:

• the location: identified by anatomy (e.g. back, chest, head, etc.)
• the quality: local, diffuse, constant, intermittent, burning, shooting, gnawing, sharp, dull, throbbing, etc
• radiation: the distribution from the place where it is felt worst
• frequency (continually, occasionally, etc.)
• triggering agents: exercise, emotional state, ingestion of certain food, etc.
• associated symptoms: shortness of breath (sometimes with chest pain), photophobia (often with migraine), bloating (often with gallstones), etc.

9. palate (adj. palatal)
Author: Guo Li
Definition:

The palate is the tissue that separates the nasal cavities and the oral cavity; it forms the floor of the nasal cavity and the roof of the oral cavity.

10. palea
Author: Mirrored from Jim Croft's Flora of Australia
Definition:

In a grass floret, the upper one of the two bracts enclosing a flower.

11. paleontology
Definition:

The scientific study of extinct organisms through the examination of fossils.

12. palindrome
Definition:

1. A word or sentence that reads the same backwards as it does forward, such as "radar"
    
2. A nucleic acid sequence whose 5'-to-3' sequence is identical on each DNA strand. The sequence is the same when one strand is read left to right and the other strand is read right to left. Recognition sites of many restriction enzymes are palindromic.

13. palliative
Definition:

1. Noun: something aimed at relieving symptoms.
2. Adjective: describes a therapy aimed at relieving the symptoms of a condition; for incurable conditions (such as some cancers), pallitative therapy (such as administering pain relief) is the primary medical treatment a patient receives.

14. pallor
Author: Guo Li
Definition:

Having a pale color.

15. palmate
Author: Mirrored from Jim Croft's Flora of Australia
Definition:

Of a leaf, divided into several leaflets which arise at the same point (as with a leaf from a palm tree).

16. palmatifid
Author: Mirrored from Jim Croft's Flora of Australia
Definition:

Of a leaf, deeply divided into several lobes which arise (almost) at the same level. Compare pinnatifid.

17. palmatisect
Author: Mirrored from Jim Croft's Flora of Australia
Definition:

A condition intermediate between palmate and palmatifid, with the green tissue of the lamina completely divided into several segments, but the segments not fully separated at the base.

18. palpation (palpate, adj: palpable)
Author: Guo Li
Definition:

Palpation is a method of clinical examination in which the examiner attempts to feel the condition of the abdominal organs by applying his/her hands on the abdominal wall.

19. palpitation
Author: Guo Li
Definition:

Palpitation is a sensation in the chest which is caused by irregular or rapid cardiac contractions.

20. palsy
Definition:

Another term for paralysis (the inability to move a body part that is normally moveable via conscious effort).

21. palustrine
Author:
Definition:

Living or thriving in a marshy environment; being or made up of marsh.

22. palynology
Definition:

The scientific study of fossil spores, pollen, and microscopic organisms preserved in sedimentary rock.

23. Pancoast's tumor
Author: Guo Li
Definition:

Pancoast's tumor is a tumor in the apex of the lung or superior pulmonary sulcus.

24. pancreas
Author: PharmInfoNet
Definition:

An organ behind the lower part of the stomach that is about the size of a hand. It makes insulin so that the body can use glucose (sugar) for energy. It also makes enzymes (trypsin, chymotrypsin, and carboxypolypeptidase) that help the body digest food. Spread all over the pancreas are areas called the islets of Langerhans. The cells in these areas each have a special purpose. The alpha cells make glucagon, which raises the level of glucose in the blood; the beta cells make insulin; the delta cells make somatostatin. There are also the PP cells and the D1 cells, about which little is known.

25. pancreatitis
Author: PharmInfoNet
Definition:

Inflammation (pain, tenderness) of the pancreas; it can make the pancreas stop working. It is caused by drinking too much alcohol, by disease in the gallbladder, or by a virus.

26. pandemic
Definition:

An epidemic that affects a wide geographic area.

27. pandurate
Author: Mirrored from Dave Sutton's Aquatic Plant Glossary
Definition:

Shaped somewhat like a violin, as some leaves.

28. panicle
Author: Mirrored from Jim Croft's Flora of Australia
Definition:

A compound raceme; an indeterminate inflorescence in which the flowers are borne on branches of the main main stem or on further branches of these.

29. paniculate
Author: Mirrored from Jim Croft's Flora of Australia
Definition: