any tool, scale, questionnaire, or procedure used to measure an outcome domain in clinical research or practice. Instruments can be:

Quantitative: e.g., blood pressure, laboratory values, physical performance tests.
Qualitative: e.g., quality-of-life measures, patient satisfaction, or other subjective experiences.

They may be disease-specific (tailored to a particular condition) or generic (applicable across multiple conditions). Organizations like OMERACT evaluate instruments to ensure they measure the intended domain accurately, detect meaningful differences, and are feasible to use.