[mhuffman]

This is interesting. In university, I was taught that a substitute product would be an alternative choice (eg. Nathan's hotdogs vs Ball park franks) and complimentary products were ones that typically "moved" together (eg. Nathan's hotdogs and ketchup). So not really opposite unless you are talking about price vs demand.

[smallnamespace]

No, they are in fact opposed definitions if you dig in a bit.

Substitute goods have negatively correlated demand, while complementary ones have demand that positively correlate. A simple criterion, at least in theory. You will buy more ketchup when you buy more franks, but you will buy fewer hot dogs.

And it's also close enough to the MBA definition of complement (of "commoditize your complement" fame).