By my definition, that's not an investment. Nor would buying a rare painting be an investment, but speculation. By buying gold, or a rare painting, and keeping it locked away in your house, you're not making capital available for productive use. You're just speculating that its price, as measured in dollars, will increase. A transfer of value from the next buyer to yourself happens when you sell it, but no value has been produced.

Regardless of the terminology we choose, can't we agree there's a fundamental difference between buying something with money and holding on to that, versus making that money available for productive use (e.g. a producer buying more efficient machinery)? When you buy a bond you're investing, because the issuer can use your capital to increase its productivity, while paying out a part of the resulting profits as a yield. When you buy a lump of gold and gold on to it, all you're hoping for is that the dollar will be devalued sufficiently to make it appear that you can sell it for a profit (in dollars). No increase in productivity needs to take place for the latter to occur, whereas in the former case bond issuers can't afford to pay interest without creating profits.