Preferences that have a single maximum over a linearly ordered set of alternatives. A set of alternatives can be linearly ordered if it is possible to arrange them along a one-dimensional axis. For example, the location of shops on one side of a street is linearly ordered. Preferences are single-peaked if there is a unique most-preferred point on the axis, and the valuation of other alternatives declines monotonically away from the mostpreferred point in either direction. This means that any individual asked to choose or vote will always prefer the most-valued level to any other, and, of any two alternatives on the same side of the most preferred value, will vote for that nearer the preferred value.
Reference: Oxford Press Dictonary of Economics, 5th edt.