Things that are added to make something longer, larger, or more complete; additions to an original structure or idea.
From Latin 'extendere' meaning to stretch out, combined with the noun suffix '-ion' and then pluralized. The meaning evolved from literal stretching to anything that expands or prolongs something.
Your hair extensions, a browser extension, and a deadline extension all use the same fundamental idea—they're all ways of making something reach further than it naturally would, which is why the word works across so many different contexts.
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