To be watchful is to be alert and aware. A watchful dog might keep an eye on the sidewalk in front of your house all afternoon.

The adjective watchful is good for describing someone who keeps an attentive eye on things. A watchful teacher, for example, knows immediately when a student is being teased, and a watchful driver is quick to stop for a pedestrian crossing the street. When you're on the lookout, you're being watchful. The word comes from the Middle English waccheful, which used to have an alternate meaning of "wakeful" or "sleepless."

Definitions of watchful
  1. adjective
    engaged in or accustomed to close observation
    synonyms: alert
    argus-eyed, open-eyed, vigilant, wakeful
    carefully observant or attentive; on the lookout for possible danger
    (British informal) not to be deceived or hoodwinked
    heads-up, wide-awake
    fully alert and watchful
    lidless, sleepless
    always watchful
    marked by keen caution and watchful prudence
    not in a state of sleep; completely conscious
    full of life and energy
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    unalert, unvigilant, unwatchful
    not alert to what is potentially dangerous
    not alert to danger or deception
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  2. adjective
    experiencing or accompanied by sleeplessness
    “"twenty watchful, weary, tedious nights"- Shakespeare”
    synonyms: insomniac, sleepless
    not in a state of sleep; completely conscious
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