The quality of being alert or on the alert; briskness; nimbleness; activity.(noun)
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Use "alertness" in a sentence
"The medium/high caffeine consumers who got the placebo reported a decrease in alertness and increased headache, neither of which were reported by those who received caffeine."
"Unfortunately, the placebo takers also had unmistakable headaches and, temporarily, occasional decreases in alertness or energy."
"The undead we encountered were a nice exercise in alertness, because some of them were invisible, and as we fought our way through them up to the parapet where the questgiver waited for us, I admired our solid teamwork."