Opportunism - the policy or practice, as in politics, business, or one's personal affairs, of adapting actions, decisions, etc., to expediency or effectiveness regardless of the sacrifice of ethical principles.
The above is how I use the term. Has it taken on any different meaning within Marxist circles? This is something that's unclear to me.
Here is Lenin's definition of "opportunism", which I think is much clearer and sharper than non-Marxist usage:
“The opportunist does not betray his party, he does not act as a traitor, he does not desert it. He continues to serve it sincerely and zealously. But his typical and characteristic trait is that he yields to the mood of the moment, he is unable to resist what is fashionable, he is politically short-sighted and spineless. Opportunism means sacrificing the permanent and essential interests of the party to the momentary, transient and minor interests.” —V. I. Lenin, “The Russian Radical is Wise After the Event” (Oct. 18, 1906), LCW 11:239.