乙部 (yǐ bù) literally means 'Section B' or 'Division B', where 乙 (yǐ) is the second character in the traditional Chinese Heavenly Stems (a ten-term cycle used for counting, dating, and classification), and 部 (bù) means 'section', 'division', or 'department'. Though historically tied to calendrical and philosophical systems, 乙部 is most commonly used today in modern contexts like administrative organization, textbook chapter labeling, or biological taxonomy—especially in zoology, where it may denote a secondary taxonomic division within a larger group.

In biology textbooks or field guides, 乙部 often appears as a formal subcategory—for example, grouping certain mammals or birds under 'Section B' when contrasting with 甲部 (jiǎ bù, Section A). It carries a formal, academic tone and is rarely used in casual speech. Learners should note that 乙部 does not refer to a specific animal but rather functions as a structural label, much like 'Appendix B' or 'Chapter 2' in English documents.