A teardrop is one single tear — one drop of the salty water that comes from your eye when you cry. English already has the word 'tear' for this, so why have 'teardrop' too? Because it makes the single drop feel separate and visible: you use it when you want the reader to see one drop, on its own, on a cheek. That makes it a rather written, slightly poetic word — songs and stories are full of teardrops, everyday conversation mostly is not. Ordinary speech says 'She had tears in her eyes', not 'She had teardrops in her eyes'. It has a second life as a shape word. A teardrop shape is round at the bottom and comes to a point at the top, the way a falling drop looks. So you get teardrop earrings, a teardrop-shaped table, a teardrop caravan. In that use there is no sadness at all — it is purely about the outline.