I was going to respond to someone's reference to a "poly lifestyle" today, as a matter of fact. It actually made me go and look up the word lifestyle, just to make sure I wasn't overreacting.
According to Dictionary.com: "the habits, attitudes, tastes, moral standards, economic level, etc., that together constitute the mode of living of an individual or group."
From my trusty ol' American Heritage dictionary: "A way of life or style of living that reflects the attitudes or values of a person or group."
Basically, it's a manner of living, but there are so many varying lifestyles of people who practice polyamory that it truly is impossible to define what a polyamorous lifestyle would be. How poly is practiced depends on the individuals involved, and there is no One True Way and, therefore, no poly lifestyle.
I often do hear people refer to poly as a "lovestyle," though. What do you think of that term? It kinda bugs me, too.