The most typical definition of an asset class is a group of securities that have similar risk/return characteristics, and behave similarly in the marketplace. Thus, for instance, stocks, bonds, and cash represent the three most common asset classes, as each have different risk/return characteristics and they behave very differently in response to various economic and market events.
One of the most common ways to attempt to determine whether an investment represents a unique asset class is to examine its correlation with other investments. After all, two investments that have different risk/return characteristics and behave differently in response to market events would likely show little similarity in returns over time, thereby exhibiting a low correlation. In turn, given how Modern Portfolio Theory demonstrates that investments with a low correlation to the rest of the portfolio can lower the overall volatility of the portfolio - even if the underlying investment itself is volatile on its own - advisors have increasingly sought out low correlation "alternative" asset classes and investments to manage risk through diversification.