Dictionary.com's Word of the Year is...
"period between ancient and modern times" (formerly roughly 500-1500 C.E., now more usually 1000-1500), attested from 1610s, translating Latin medium aevum (cf. German mittelalter, French moyen âge).
"period between youth and old age," late 14c.; middle-aged (adj.) first recorded c.1600.
middle age mid·dle age (mĭd'l)
The time of human life between youth and old age, usually reckoned as the years between 40 and 60. Also called midlife.
The period of European history between ancient and modern times. The Middle Ages began with the Fall of Rome in the fifth century and ended with the Renaissance. The Middle Ages are associated with many beliefs and practices that now seem out of date, such as chivalry, feudalism, the Inquisition, the belief that the sun revolves around the Earth, and a host of popular superstitions. The early Middle Ages are even sometimes called the Dark Ages. The Middle Ages, however, especially in later years, also saw many notable human achievements. Among these were the building of modern nations, such as England and France; increasingly sophisticated and expanded trade; a great advancement of technique in philosophy and theology; some remarkable works of literature (see The Canterbury Tales, The Divine Comedy); and the building of magnificent churches (see Chartres and Notre Dame de Paris).
period of human adulthood that immediately precedes the onset of old age. Though the age period that defines middle age is somewhat arbitrary, differing greatly from person to person, it is generally defined as being between the ages of 40 and 60. The physiological and psychological changes experienced by a middle-aged person centre on the gradual decline of physical abilities and the awareness of mortality. In middle age, the relative potencies of past, present, and future are altered as the individual increasingly directs effort to the process of reminiscence and recollection of the past, rather than anticipation of the future. If approached constructively, middle age can prepare an individual for a satisfying and productive old age. See also psychological development.