<p><strong>noun</strong></p><ul><li>a person or thing that watches or stands as if watching.</li><li>a soldier stationed as a guard to challenge all comers and prevent a surprise attack.</li><li>a bone in the human leg extending from the pelvis to the knee, that is the longest, largest, and strongest in the body; thighbone.</li><li>an indication or mark that a disease is present or prevalent.</li></ul><p><strong>verb</strong></p><ul><li>to watch over or guard as a sentinel.</li></ul><p><strong>adjective</strong></p><ul><li>relating to or being an indication of a disease's presence or prevalence.</li></ul>