DD has several words for it. She calls it milk and will point at her nursing friends and pictures of nursing animals in books and such and say milk. We also went through a phase where she was pointing at other women's breasts and saying milk.
Fortunately she only did it with nursing moms, even if they weren't actively nursing at the moment. We're pretty fortunate in that DD is surrounded by nursing toddlers
For awhile she was asking for milk when she wanted to nurse. Lately she's asking for side. It's my fault as I'm always saying "other side?", "you may only have one side at a time," "Do you want this side or that side?" etc. I think she's actually calling my breasts sides. I have not corrected her with the term breast (not what I want my toddler screaming in public), but I have encouraged her to say nurse or milk. She does say the word nurse, pretty well too, but not generally when asking for it.