In an economic sense,
An INVESTMENT is the purchase of goods that are not consumed today but are used in the future to create wealth.
In finance
An INVESTMENT is a monetary asset purchased with the idea that the asset will provide income in the future or will later be sold at a higher price for a profit.
I really don't want to turn these into an economics class but the both definition of what investments are clearly should settle the argument of whether BTC is truly an investment or not.
To be honest, if you ask me about investments, as long as I have placed my own money to it and envisioning it to gain value in the future, then that's a good enough investment for me. I casually invest in bitcoins and I don't really invest most of my money on it but I'm glad it has grown already.