A small problem or irregularity that doesn't last long; also, a spot of light on a radar screen.
From the 1940s, likely onomatopoetic—imitating the sound of a radar signal. The word probably comes from the 'blip' sound radar equipment made.
During World War II, radar was brand new technology, and the British and Americans literally invented the word 'blip' to describe what they saw on screens—it's a perfect example of onomatopoeia creating technical vocabulary.
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