Zhège língjiàn de biǎomiàn yǒu jǐ tiáo píngxíng de āoxiàn.
The surface of this part has several parallel grooves.
Tips
memory
凹 is one of the most pictographic characters in Chinese - the shape itself looks like a depression. Pair it with 凸 (convex / bulging out) and you have a perfect visual pair.
