opportunist

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. One who takes advantage of any opportunity to achieve an end, often with no regard for principles or consequences.

Century Dictionary and Cyclopedia

1. n. [capitalized] In French politics, a member of that section of the Republican party which believes in regulating political action in accordance with circumstances, and not by dogmatic principles. This word first came into use in France about 1873. The Opportunists were the party of concession, and occupied an intermediate position between the various groups of monarchists and the Intransigentists, the extreme section of the Republican party. Their leader was Gambetta.
2. n. In general, one who takes advantage of opportunities as they occur; one who waits for an opportune time before attempting to bring into practice or to urge upon others the principles or beliefs which he holds; one who makes the best of circumstances as they arise; hence, one who is without settled principles or consistent policy: opposed to extremist.
3. Of, pertaining to, or characteristic of the political party known as the Opportunists; hence [lowercase], of or pertaining to opportunism, or the observance of a waiting policy; making the best of circumstances while waiting for a suitable time for the proper carrying out of one's views.

Wiktionary

1. n. Someone who takes advantage of any opportunity to advance his own situation, placing expediency above principle.

GNU Webster's 1913

1. n. Recent One who advocates or practices opportunism.

WordNet 3.0

1. adj. taking immediate advantage, often unethically, of any circumstance of possible benefit
2. n. a person who places expediency above principle