marker
US /ˈmɑːr.kɚ/
UK /ˈmɑːr.kɚ/

1.
a felt-tip pen with a broad tip, used for drawing or writing
:
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Please use a permanent marker for the labels.
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The teacher wrote on the whiteboard with a blue marker.
2.
an object used to indicate a position or place
:
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Place a marker at the starting line.
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The buoy serves as a marker for the safe channel.