heart of string
Pictures with a line that forms a heart-shaped loop. It's not really important if it's supposed to be an actual string, it may be a ribbon, a typographical element or a background decoration, etc.
The equivalent Pixiv tag is ラインのハート, literally "heart made of a line".
Komeiji Satori and her sister Komeiji Koishi sport these frequently.
See also
This tag implicates heart (learn more).
