A standard or regular serving/portion (as on menus); commonly contrasted with larger portions like 'large serving' (大盛).
牛丼は並盛でお願いします。
I'll have a regular-sized beef bowl, please.
並盛と大盛の違いはご飯の量だけですか？
Is the only difference between the regular and large serving the amount of rice?
この店の並盛は女性でも食べ切れる量だ。
The regular serving at this shop is an amount even women can finish.