脑补 (Nǎo bǔ)
This is a very interesting term. It refers to brain supplements but it is not usually used in everyday conversations. Instead, it is usually used in comics, or novels to describe a character being upgraded in their mental state. However, we know that these can't happen in real life but such fantasies are possible, and also commonly used as plot points in stories.
One example is the famous movie, Lucy, starring Scarlett Johansson who took some drugs which caused her brain power and capacity to improve tenfolds. This is what the Chinese would describe as 脑补.
It can also be used to describe your own fantasies.
For example:
"玩游戏自己想象剧情也是脑补"
(Wán yóuxì zìjǐ xiǎngxiàng jùqíng yěshì nǎo bǔ)
Which means: If you play a game and imagine different scenarios is also a form of 脑补!