A steamed bun filled with meat (nikuman). In Japan it’s a type of Chinese-style steamed bun commonly sold at convenience stores in winter; this is the colloquial/short form.
冬になるとコンビニで温かい肉まんが売られる。
In winter, warm nikuman are sold at convenience stores.
朝ごはんに肉まんをひとつ買って食べた。
I bought and ate a nikuman for breakfast.
この店の肉まんは具がたっぷりで人気がある。
This shop’s nikuman are popular because they have plenty of filling.