Plain English Meaning
A small filled-in triangle on a wind symbol that stands for 50 knots of wind. You add it together with the lines on the same symbol to get the total wind speed.
Full Definition
On a surface analysis chart, a solid filled triangle (flag-shaped symbol) attached to a wind barb that represents 50 knots of wind speed. Pennants are combined with full barbs (10 knots) and half barbs (5 knots) on the shaft to indicate total wind speed at the reporting station.
Context Anchor
Seen on weather chart station plots, where wind direction and wind speed are shown with marks attached to the wind symbol.
Derivation
From the Latin penna, meaning 'feather' or 'wing,' which became 'pennant' for a small triangular flag. On a wind barb, the symbol looks like a tiny triangular flag attached to the shaft — hence the name.