Yes and no......
Technically a "weed" is something which is capable of rapid proliferation and hard to eradicate. MJ is that kind of plant.
The police have not been able to stamp out MJ....
The original idea behing 420 was for everyone to save their seeds up over the year and on 4/20 .. go out and spread them along the highways and fields everywhere, and overwhelm the cops and DEA. Of course stoners are easily distracted and the real message was lost. It would work though.
BUT....
On the other hand, botanically.... a "weed" is something man has not found a use for, or not enough of a use to put it into production. A weed is typically just a misplaced plant. Some plants are designed to be pioneering plants. In floriduh, there is a plant called red root. It seems to be a weed, but studies have shown that while it takes a disturbed area (man creates
), it actually stabilizes the soil and gives way when other species recover. it's a tricky thing to say "this is the way it is" in science.... new data is always coming in.
So is weed a "weed"? Yes, I think it qualifies, although man has found a use for it. MJ is a weed botanically. It may be a "pioneering" plant as well. That part I have not studied.