'奇数' (qí shù) literally combines 奇 (qí), meaning 'odd' or 'strange', and 数 (shù), meaning 'number'. In mathematics, it specifically refers to any integer that cannot be evenly divided by 2 — such as 1, 3, 5, −7, or 101. Though 奇 can mean 'strange' in other contexts, here it carries its classical mathematical sense of 'not divisible by two', rooted in ancient Chinese numerology and logic.

This term is used formally in math education, programming, data analysis, and everyday counting tasks — for example, when sorting items, checking IDs, or describing patterns. It contrasts directly with 偶数 (ǒu shù, 'even number') and appears frequently in textbooks, exams, and technical instructions. Learners should note that 奇数 is always a countable noun and never used attributively without a classifier or measure word in formal writing.