Ida Greene is the mother of Dr. Leonard Greene and the paternal grandmother of Amy, Rachel and Jill Greene. This relationship makes her the great-grandmother of Rachel's daughter, Emma.
History
She is an unseen character on the show; she is mentioned in "The One With Rachel's Phone Number" when Phoebe and Rachel go out to a bar and meet two guys who live in her building.
Apparently, she smells like potatoes so the people who lived in her building nicknamed her "Spuds".