Panties that are visible beneath one's own pantyhose.
Note that this tag should generally not be used in cases where the panties are only partially covered by pantyhose, such as in post #792837.
The equivalent pixiv tags are タイツ越しのパンツ, ストッキング越しのパンツ, and パンスト越しのパンツ. The terms have slight differences, but can be summarized as:
ストッキング ("stockings"): generic term for any kind of stockings or pantyhose.
パンスト ("panty-stocking"): original term for sheer pantyhose and associated with OLs and flesh-tone pantyhose. The term is somewhat dated and not really used by young women anymore.
タイツ ("tights"): essentially dark or slightly thicker pantyhose. Nowadays the most popular variant and also widely the modern term for "pantyhose".