Fundamental question: What's a pixel?

Ok I know this is very elementary but after I read an article I got confused. What is considered a pixel?

1. Combination of (R+G+B) photosite?
2. Each R and G and B photosite.

So when you have 10 Megapixel, does it mean total R+G+B pixel =10Mp or is there 3x10Mp (or is it 4x10Mp?) photosites?

When they talk about pixel size, are they referring to the combined RGB pixel or just each photosite either R or G or B? Then you have to take into account there are 2 (G)reen pixels vs 1 for R and B to compensate for the sensitivity of Human Eye to Green. Looking at Bayer Pattern layout you would think pixel size refers to each photosite...What gives?
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Alex