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Also actini‑.

A ray or beam.

Greek aktis, aktin‑, ray.

Some examples refer to rays of light. Radiation that is able to cause chemical reactions, as light does on photographic film, is said to be actinic; an actinometer is an instrument for measuring the intensity of radiation. Actinium is a rare radioactive element (atomic number 89), so named because it glows in the dark; hence actinide, a member of the series of radioactive elements, including uranium and plutonium, of which actinium is the first.

Other examples refer to structures that radiate from a centre. Something actinomorphic, such as a starfish or the flower of a daisy, has radial symmetry. An actinomycete (Greek mukēs, mukēt‑, fungus) is a bacterium with a filamentous form, which was formerly considered to be a fungus, the ray-fungus; these can cause disease in humans and animals; actinomycin is a member of a group of antibiotics extracted from soil bacteria of this genus.

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