永 is mainly seen in 永远 (forever/always). Also: 永久 (permanent), 永恒 (eternal).
history
永 is a famous character in Chinese calligraphy - the 永字八法 teaches that this single character contains all eight basic brush strokes needed for writing.
The water radical supplies the four-stroke body beneath the dot. Originally 永 was a side-view pictograph of a long winding waterway; indexed under 水 because the lower body has retained the recognisable water-radical silhouette.
Single top dot functions as a positional marker indicating the source-point from which the water flows beneath. Pure abstract indicator. Historically the dot was the spring head in the original pictograph, with the strands beneath winding away as the long current that gave 永 its 'eternal' meaning.
