‘一杯’ (yī bēi) is a measure phrase meaning 'one cup' — it combines the numeral ‘一’ (yī, 'one') with the measure word ‘杯’ (bēi, 'cup'), which is used specifically for liquids or small, cup-shaped items like coffee, tea, juice, or even rice in informal contexts. Unlike English, Chinese requires a measure word between numerals and nouns, so you cannot say *‘一水’; instead, you must say ‘一杯水’ (yī bēi shuǐ, 'one cup of water').

This phrase is common in daily life — ordering drinks, cooking, giving instructions, or describing portions. ‘杯’ can also be used metaphorically (e.g., ‘一杯苦酒’, 'a cup of bitter wine'), but in most beginner contexts, it refers literally to a standard drinking cup (≈150–250 ml). It’s neutral in register — appropriate in casual speech, restaurants, recipes, and health-related advice.