A 4Pin 2016 2.0x1.6mm SMD Passive Crystal Oscillator is a tiny, surface-mount electronic component (2.0mm x 1.6mm package size, often with 0.45mm height) that uses a quartz crystal to provide a precise frequency, requiring an external circuit to generate the actual signal, common in IoT, wearables, and communication devices needing stable timing. 

Applications
These are essential for timing in various modern electronics, including: 

Where to Find Them
You can find these components from manufacturers and suppliers like NDK, Chipsun, or on e-commerce platforms such as eBay, AliExpress, RS Online, and Alibaba.