According to the Mayo clinic hair loss definitioin is as follows.

Hair loss (alopecia) can affect just your scalp or your entire body. It can be the result of heredity, certain medications or an underlying medical condition. Anyone — men, women and children — can experience hair loss.
Baldness typically refers to excessive hair loss from your scalp. Some people prefer to let their baldness run its course untreated and unhidden. Others may cover it up with hairstyles, makeup, hats or scarves. And still others choose one of the medications or surgical procedures that are available to treat hair loss.
Before pursuing any treatment option, talk with your doctor about the cause of and best possible treatments for your particular type of hair loss.

Symptoms

Photograph showing male-pattern baldness (androgenetic alopecia) Male-pattern baldness
Photograph showing female-pattern baldness Female-pattern baldness
Photograph showing patchy hair loss (alopecia areata) Patchy hair loss (alopecia areata)
Photograph showing traction alopecia Traction alopecia
Hair loss can appear in many different ways, depending on the problem that’s causing it. It can come on suddenly or gradually and affect just your scalp or your whole body. Some types of hair loss are temporary, while others are permanent.
Signs and symptoms of hair loss may include:
  • Gradual thinning on top of head. This is the most common type of hair loss, affecting both men and women. In men, hair often begins to recede from the forehead in a line that resembles the letter M. Women typically retain a line of hair at the forehead but experience a broadening of the part in their hair.
  • Circular or patchy bald spots. Some people experience smooth bald spots, often about an inch (2.6 centimeters) across. This type of hair loss usually affects just the scalp, but it sometimes also occurs in beards or eyebrows. In some cases, your skin may become itchy or painful before the hair falls out.
  • Sudden loosening of hair. A physical or emotional shock can cause hair to loosen. Handfuls of hair may come out when combing or washing your hair or even after gentle tugging. This type of hair loss usually causes overall hair thinning and not bald patches.
  • Full-body hair loss. Some conditions and medical treatments, such as chemotherapy for cancer, can result in the loss of hair all over your body. The hair usually grows back after treatment ends.