What is the X in X-mas?
1942, shortening of stroboscope "instrument for studying motion by periodically interrupted light" (1896), from Greek strobos "act of whirling" + -scope.
A strobe light.
A spot of higher than normal intensity in the sweep of an indicator on a scanning device, as on a radar screen, used as a reference mark for determining the position or distance of the object scanned or detected.
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