A band worn around the leg to hold up a sock or stocking.
"She tied her silk garter tightly above her knee before stepping into her evening gown."
fasten with or as if with a garter
"She quickly tied her loose pant leg up and secured it by fastening it with or as if with a garter before climbing onto the ladder."
to fasten with a garter
"She quickly pulled up her stocking and secured it by tightening the elastic band around her ankle."