Exactly as BBP said. The word comes from:
Medieval Latin canopeum mosquito net, from Latin conopeum, from Greek kOnOpion, from kOnOps mosquito
Originally, the word referred to mosquitos. Then it was used as meaning the tented mesh that covered a bed to keep mosquitos out. Then it was used to mean anything that is "like" a canopy that covers a bed. This is when it's usage was adopted to mean the covering of foliage over a forest. Then again it was adopted to mean anything that drapes over anything, such as the canopy of a baby carriage that keeps sun off the baby. And finally, it also means the covering foliage in a group of plants of any kind and last but not least; The covering foliage of an indoor growth of marijuana plants as used by Stoney Bud.
That's more than you wanted to know, isn't it?
Picture looking down on your grow from above. The square footage of the green that you're looking at is what you want to calculate. You measure one side in feet and measure the other side in feet and multiply one by the other and you have square feet.
Let's say your crop canopy is 3 feet wide and 4 feet long.
Multiply the width times the length in feet and you have the square footage of your canopy.
That would be 12 square feet.
If you want 5,000 lumens per/sq ft of canopy, then you want 12 times 5,000 lumens.
60,000 lumens would be the amount of light you would want on your crop.
A single 430 watt HPS Hortilux LU 430S/HTL/EN bulb has 58,500 lumen output.
That would be perfect for a 3 foot by 4 foot grow. It's only 1,500 lumens short of the perfect 5,000 lumens per/sq ft.