Is nipple stimulation considered masturbation?

Hi. My girlfriend loves it when I stimulate her nippples. She says she even gets s when I do it to her. Often she would ask me to do it to her out of the blue. Anyways, I'm just curious. By stimulating her , am I masturbating her? Or can it only be called that when are involved? Is it plain ? But we don't usually do it as a prelude to actual . What do you think?