1. To reduce the size, strength, or density of something, or to become thinner, weaker, or less dense.
2. To weaken or to reduce in force, the intensity, effect, quantity, or value of something; for example, to attenuate a desire.
3. To reduce the virulence of a bacterium or virus; for example, by exposing it to heat or producing a culture of it in a special medium.

Attenuated bacteria or viruses are used in some vaccines.

4. Etymology: "to make thin, to make less" from Latin attenuatus, past participle of attenuare, "to make thin"; from ad-, "to" + tenuare, "to make thin"; from tenuis, "thin".