这个词是什么意思？
Discrete concentrations of energy, apparently massless elementary particles, that move at the speed of light. They are the unit or quantum of electromagnetic radiation. Photons are emitted when electrons move from one energy state to another. (From Hawley's Condensed Chemical Dictionary, 11th ed)
中文名称索引；定义为 MeSH 英文原文，尚未逐条译审。MESH:D017785 · MeSH 2026。
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