undercurrent

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

• noun A current, as of air or water, below another current or beneath a surface.
• noun An underlying tendency, force, or influence often contrary to what is superficially evident.

from The Century Dictionary.

• Running below or out of sight; hidden.
• noun A current in a body of water or other liquid, or in the atmosphere, below the upper or superficial currents.
• noun Figuratively, something at work below the surface or out of sight, as influence or feeling, which has a tendency opposite to or different from what is visible or apparent.
• noun In hydraul. mining, an arrangement on the sluices which is intended to aid in saving the gold.

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

• noun A current below the surface of water, sometimes flowing in a contrary direction to that on the surface.
• noun Hence, figuratively, a tendency of feeling, opinion, or the like, in a direction contrary to what is publicly shown; an unseen influence or tendency.
• adjective rare Running beneath the surface; hidden.

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

• noun a current which flows under the surface
• noun figuratively An emotional presence that is concealed rather than exposed.

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

• noun a subdued emotional quality underlying an utterance; implicit meaning
• noun a current below the surface of a fluid