1. 英语字母表的第2个字母
B is the second letter of the English alphabet.

2. C大调音阶中的第七音；B音
In music, B is the seventh note in the scale of C major.

3. （作业、考试等的）乙等，良好
If you get a B as a mark for a piece of work or in an exam, your work is good.

4. （用于首字母为b的一些单词的缩写，如born）
B or b is used as an abbreviation for words beginning with b, for example 'born'.