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Anemia
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Anemia means there are fewer red blood cells in the blood than there should be, or that hemoglobin, the substance inside those cells that carries oxygen, is at a lower concentration than expected. Expected by whose standard, though, because the number changes with age, with sex, and with a person's physiological state. That is why there is no single universal figure, the same way saying someone is tall means nothing until you know who they are being compared to.
What every anemia shares is the same physiological consequence: the blood ends up with less capacity to carry oxygen. Behind it, though, there is no single cause. Too few red blood cells may be getting made, they may be getting lost, or they may be getting destroyed faster than they should. Three different origins that end up under the same name.
In brief
Anemia is present when the red blood cell count or the hemoglobin concentration is lower than expected for a person's clinical context. That context includes age, sex, and physiologic state, so there is no single universal cutoff.
The common physiologic consequence of anemia is reduced oxygen-carrying capacity.
Anemia has many possible causes. It can result from reduced production, blood loss, or increased destruction of red blood cells.
Sources (2)
This page explains the topic in general terms; it does not tell anyone what treatment to follow.
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