The French term terroir is a slippery one. Its most basic, literal meaning in English is "soil." But its practical meaning goes well beyond any one-word definition. Terroir is not soil of the kind you transfer from plastic bag to flower pot: it is the unique soil and situation of a specific place.
For the French, it is terroir that gives value to a particular locality, setting it off from every other place and implying that a unique quality exists in that specific spot that cannot be reproduced anywhere else. Terroir is, in effect, the taste of the place.
Though the term can be applied to any agricultural product of the soil, the concept of terroir is recognized outside of France mostly in terms of wine--and what's more, for its implication of quality.
In fact, the full meaning of terroir takes on considerably more than the soil of a specific place. In terms of wine vines, terroir encompasses nothing short of the vine's total environment--and the way in which all aspects of the environment are consistently reflected in the taste of the wine. A wine from this place should taste unlike a wine from that place and we should expect this distinctiveness to be evident year in and year out.
For the French, terroir is almost spiritual. But at a more down-to-earth level, the notion of terroir can have real meaning for us in the New World, as well. Grapes are finicky fruits. They grow well in some places, poorly in others. One grape likes cool foggy weather, another likes to be smothered in sunshine. One prefers gravel, another limestone. For a grape grower, knowing where to plant vines can spell the difference between exceptional and mediocre wine.
Ultimately, we might say that terroir is the certain something that gives a wine its personality.