beacon

A signal fire to notify of the approach of an enemy, or to give any notice, commonly of warning.

(nautical) A signal or conspicuous mark erected on an eminence near the shore, or moored in shoal water, as a guide to mariners.

A post or buoy placed over a shoal or bank to warn vessels of danger; also a signal mark on land. (FM 55-501)

A high hill or other easily distinguishable object near the shore which can serve as guidance for seafarers.

(figuratively) That which gives notice of danger, or keeps people on the correct path.

An electronic device that broadcasts a signal to nearby portable devices, enabling smartphones etc. to perform actions when in physical proximity to the beacon.

(Internet) Short for web beacon.