In Websters dictionary it is defined as-the processes needed to maintain life. When you hear it being used today it is usually used in terms of weight issues.
So in most circles today you hear people say "I can't lose weight because i have a slow metabolism".
There is some truth to this-- Other factors, such as how much you eat and exercise play a much bigger role in your weight than your metabolism does.
It is true that how much lean body mass your have can affect how many calories you burn at rest, however its affect is limited. In part, because you can only build so much lean muscle by strength training.
It is how your body turns food you eat into the energy it needs! It is a vital process for all living things not just humans.
Some medications can affect your weight. A good balanced diet and regular physical activity can lessen this effect.
Eating Breakfast every day can get your metabolism off to a good start.
Age can slow your metabolism . As a person ages you tend to gain fat and lose muscle. Some people also become less active. So as you age , you need to do more to keep your metabolism up.