1. A barrier surrounding a region to exclude it from the influence of an energy field. For example, to protect a region from an electric field an earthed barrier is required; to protect it from a magnetic field a shield of high magnetic permeability is needed.
2. A barrier used to surround a source of harmful or unwanted radiations. For example, the core of a nuclear reactor is surrounded by a cement or lead shield to absorb neutrons and other dangerous radiation.
3. (in atoms) The barrier provided by inner electron shells to the influence of nuclear charge on outer electrons. Shielding has an effect on ionic radius, as in the lanthanoids.
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