Question: According to Android definitions, I've found that:

* Resolution: The total number of physical pixels on a screen
* Density: Based on the screen resolution, the spread of pixels across the physical width and height of the screen.

How can I understand better the differences between the two definitions?

Answer: 
Basically:

1. **Resolution** concerns an *absolute* number of pixels. (check out wikipedia [Image Resolution](http://en.wikipedia.org/wiki/Image_resolution))
2. **Density** (aka Pixels per inch - PPI) concerns a *relative* number of pixels per inch. (check out the wikipedia [Pixel Density](http://en.wikipedia.org/wiki/Pixel_density))

Take care,
Beco
**Resolution** is the number of pixels on a device found in each dimension (width × height) that can be displayed on the screen. 

*For example,* a device with the resolution of “1024 × 768” has a 1024-pixel width and a 768-pixel height.

**Pixel Density** is usually measured in PPI (Pixels Per Inch), which refers to the number of pixels present per inch on the display. A higher pixel density per inch allows for more sharpness and clarity when using the device.

*For example,* If a device screen is called "100 PPI" then it means that it has 100 psychical pixels per inch.

[Reference](https://blog.prototypr.io/making-sense-of-device-resolution-pixel-density-40922aeb8a6)