Hypertension is the medical term use for high blood pressure.
The definition of what constitutes high blood pressure, and how and when it should be treated, is contained in "the Seventh Report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure (JNC 7)". JNC is a committee of the National Heart, Lung, and Blood Institute.
Systolic blood pressure: The pressure in the arteries generated by the contraction of the left ventricle, the major pumping chamber of the heart. When the heart pumps, the pressure rises.

Diastolic blood pressure: The pressure in the arteries while the left ventricle fills, before pumping again. Because there is a valve between the left ventricle and the arteries, the pressure never drops to zero. When the left ventricle "rests", the pressure is lower.

Normal blood pressure: Less than 120 mmHg systolic, and less than 80 diastolic.

Pre-hypertension: 120-139 systolic, or 80-89 diastolic.

Stage I hypertension: 140-159 systolic,  or 90-99 diastolic.

Stage II hypertension: Greater than or equal to 160 systolic, or greater than or equal to 100 diastolic.
When I was in medical school, someone whose systolic blood pressure was greater than 140, and less than 160, was not treated with medication. We asked the patient to exercise, cut out salt in their diet, and perhaps lose weight. Today we know that the patient should not exercise until their blood pressure is under control. Today we know that we prolong the patient's life by controlling their blood pressure. Therefore, we are much more aggressive about treating with medication early.

Diet: Doctors have long asked patients with hypertension to avoid excessive salt. A general rule of thumb is to not salt at the dinner table, not add salt when cooking, and avoid foods made by soaking them in salt (dill pickles, corned beef, etc.). Recently, a dietary approach referred to as DASH (the Dietary Approach to Stopping Hypertension), has been shown in studies to markedly reduce high blood pressure. More studies need to be done.
An excellent source of information cncerning diet and hypertension is the NHLBI; ee www.nhlbi.nih.gov/hbp/prevent/sodium/sodium.htm

Exercise: Exercise has many advantages for the hypertensive patient. We prefer to have the patient's blood pressure under control before the patient embarks on a new exercise program. It may also be appropriate for the patient to have an exercise treadmill test done before starting an exercise program.

Alcohol intake: Excessive alcohol intake on a regular basis raises the blood pressure. Moderate use (2 or fewer drinks per day) normally does not raise the blood pressure.

Medications: When I was in medical school, high blood pressure medicines were taken 4 times a day. They often caused patients to feel lightheaded and dizzy. They often caused dry mouth. Usually, if a man took them, it negatively affected his ability to enjoy intimacy with his wife. Therefore, compliance on the part of the patient in taking their medicine was understandably bad. High blood pressure was not well-controlled.

Today though, we have inexpensive, generic blood pressure medicines that are taken once a day and rarely cause side effects. I have never encountered a patient's high blood pressure that could not be controlled eventually.

Treat not cure: We treat high blood pressure, we do not cure it. Once the patient takes medicine for high blood pressure and it comes under control, it does not mean we can stop it. In fact, suddenly stopping high blood pressure medicine can be dangerous.
Coronary artery disease
Hypertension is a major risk factor for coronary artery disease (the cause of heart attacks, and the #1 killer of men and women in America). Of those risk factors which we can modify, the four most important are: Hypertension, high cholesterol, diabetes, and tobacco use. If the patient has high blood pressure, it is critical that they never use tobacco. If they are already using it, they must quit. The patient should be screened for high cholesterol and high blood sugar. If either is a problem, they should be treated, and controlled.