De Quervain's tenosynovitis is a condition where the tendons on the thumb side of the wrist become swollen or irritated. It causes pain during thumb motion and is usually seen in middle-aged women. It occurs when repetitive motions like assembly line work are performed. A positive Finkelstein's test, where pain is felt when the thumb is pulled across the palm, can help diagnose the condition. Treatment focuses on reducing inflammation and bracing the wrist during activities, and the prognosis is generally good with resolution of symptoms.