[jonnycat]

Right - the word "hallucination" is used a lot like the word "weed" - it's a made-up thing I don't want, rather than a made-up thing I do want.

[codetrotter]

How is weed made up? Isn’t it just dried leaves from the cannabis plant?

[Y_Y]

OP mostly likely means "weed" like "pest" or "annoyance", i.e. a category of undesirable plants that tend to appear unbidden along with desirable plants. The distinction isn't biological, it's just that when you create a space for growing then things that grow won't all be what you want.

(The term "weed" for marijuana is just a joke derived from that sense of the word.)