Carefully observing and paying close attention to something, especially for safety or to notice any changes.
From 'watch' (Old English 'wæccan,' to wake or observe) plus the adjectival suffix '-ful,' meaning full of or characterized by. The word has meant attentive observation since medieval times.
Watchfulness is neurologically different from just seeing—it involves sustained attention, which is controlled by different brain regions than casual vision, which is why a watchful guard can notice a small change that someone casually glancing would completely miss.
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