erratic

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Having no fixed or regular course; wandering.
• adjective Lacking consistency, regularity, or uniformity.
• adjective Deviating from the customary course in conduct or opinion; eccentric.
• noun A rock fragment that has been transported by ice to a location other than its place of origin and that may range in size from a pebble to a large boulder.

from The Century Dictionary.

• In zoology, occurring in an unusual location.
• Wandering; having no certain course; roving about without a fixed destination.
• Deviating from the proper or usual course in opinion or conduct; eccentric.
• Moving; not fixed or stationary: applied to the planets as distinguished from the fixed stars.
• In medicine, irregular; changeable; moving from point to point, as rheumatic or other pains, or appearing at indeterminate intervals, as some intermittent fevers.
• In geology, relating to or explanatory of the condition and distribution of erratics. See II., 2.
• noun One who or that which has wandered; a wanderer.
• noun Specifically In geology, a boulder or block which has been conveyed from its original site, probably by ice, and deposited at a distance; an erratic block. See erratic blocks, under I.
• noun An eccentric person.

from the GNU version of the Collaborative International Dictionary of English.

• noun One who deviates from common and accepted opinions; one who is eccentric or preserve in his intellectual character.
• noun obsolete A rogue.
• noun (Geol.) Any stone or material that has been borne away from its original site by natural agencies; esp., a large block or fragment of rock; a bowlder.
• adjective Having no certain course; roving about without a fixed destination; wandering; moving; -- hence, applied to the planets as distinguished from the fixed stars.
• adjective Deviating from a wise of the common course in opinion or conduct; eccentric; strange; queer.
• adjective Irregular; changeable.
• adjective (Geol.) masses of stone which have been transported from their original resting places by the agency of water, ice, or other causes.
• adjective the phenomena which relate to transported materials on the earth's surface.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective unsteady, random; prone to unexpected changes; not consistent
• noun geology A rock moved from one location to another, usually by a glacier.
• noun Anything that has erratic characteristics.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adjective having no fixed course
• adjective likely to perform unpredictably
• adjective liable to sudden unpredictable change