Generically, “speculation” is the purchase of an asset with the hope that it will become more valuable in the future. Speculation tends to imply that the holder of the asset is relying on price movements or other externalities to generate profit, as opposed to investing money in the asset to improve the asset — these are not absolutes, however. For example, house flippers are often termed speculators but they do generally invest in the properties (but with an eye toward boosting short-term value and “curb appeal”).
There is also an element of “arbitrage” at play in the domain space — taking advantage of price differences in different areas of the market, or knowledge (e.g., “business intelligence”) gaps between different parts of the market. An arbitrageur tends to take advantage of specific, often short-term, circumstances in the market to create an exploitable gap between the price certain sellers are willing to accept (or must accept) and the price certain buyers are willing to pay. The ability to buy non-fungible assets (like domain names) at a low fixed price and then sell some of those assets at significantly higher prices (because their value to the second buyer has little relationship to the price charged by the first seller) creates an opportunity akin to arbitrage.
By contrast, a speculator tends to count on broader market movements (i.e., the tendency of markets to rise over time (while avoiding the need to sell in a short-term market dip).