适应 (shì yìng) is a verb meaning 'to adapt' or 'to adjust' — especially to new, changed, or challenging conditions. The first character 适 (shì) means 'suitable' or 'fitting', and the second 应 (yìng) means 'to respond' or 'to comply'. Together, they convey the idea of actively responding in a way that makes one fit or suitable for a new environment or situation. It’s commonly used in contexts involving biological adaptation, social adjustment, learning new routines, or coping with change.

This word often appears in formal or semi-formal registers — in science textbooks (e.g., animal behavior), workplace training materials, or discussions about immigration, education, or health recovery. While it can be used reflexively (e.g., 他需要时间适应), it’s rarely used as a noun; instead, the noun form is 适应性 (shì yìng xìng, 'adaptability'). It implies conscious or physiological effort, not passive acceptance.