A minimally invasive diagnostic procedure in which a very fine needle (thinner than a blood draw needle) is inserted into a mass or suspicious lesion under ultrasound guidance or palpation, to withdraw (aspirate) a sample of cells and fluids. This sample is then spread on microscopic slides, stained, and examined by a pathologist (cytopathologist) to determine the pathological nature of the cells (normal, benign, malignant).