: a person who thinks or guesses especially in an idle or casual way about something that is unknown or uncertain
I sat Friday night in the dining room in front of my laptop, the TV in the next room flitting among experts and speculators about what happened in Paris … Scott Martelle
"… All that we ought to ask, therefore, is, that the witnesses of our conduct, and the speculators on our motives, should be capable of taking the highest view which the circumstances of the case may admit. …" Nathaniel Hawthorne
b
: a person who makes a relatively risky investment in something (such as stocks or real estate) in the hope of making a large short-term profit from market fluctuations
futures/currency speculators
By the late 1870s Edward Pray was living in New York City, still interested in mining but instead now as a speculator in mining stocks. Willa Kane
Over the years, many houses have been bought by speculators or real estate companies and rented out. Margaret Gillerman
Word History
Etymology
speculate + -or entry 1; in earlier sense "observer, lookout," borrowed from Latin speculātor "scout, spy, sentinel," from speculārī "to keep a close watch on, spy out, watch for" + -tor, agent suffix — more at speculate