笔(bǐ) - pen in Mandarin

笔 (bǐ) is the general word for a pen, and more broadly for any hand-held writing instrument: pens, pencils and brushes are all a kind of 笔 (bǐ). On its own it usually defaults to "pen" in modern everyday speech, especially a ballpoint or fountain pen.

It is also the root of a whole family of writing words: 铅笔 (qiānbǐ), "pencil", literally "lead pen", 毛笔 (máobǐ), "calligraphy brush", literally "hair pen", and 钢笔 (gāngbǐ), "fountain pen", literally "steel pen". Learning 笔 (bǐ) first makes all of those transparent later.

The measure word is 支 (zhī), the one used for long thin objects: one pen is 一支笔 (yì zhī bǐ).