Deriving from ye'da (knowledge), de'ah in today's Hebrew means opinion. Thus you might find yourself agreeing with a ma'amar de'ah - an opinion piece - you read in the newspaper.
In the Bible, de'ah means knowledge. Isaiah prophesies, "For the land will be filled with de'ah of God" ((11:9). Elsewhere, Isaiah laments the absence of anyone interested in learning de'ah: "Whom shall I teach de'ah?" (28:9). Here, it seems that de'ah implies a consciousness of God. Because society has become hedonistic and idolatrous, Isaiah has no one to whom to teach God's message.
Pirkei Avot uses de'ah differently and speaks of four types of de'ot, meaning temperaments: "Easily angered and easily pacified . . . one who is hard to anger and hard to pacify etc" (5:14). Maimonides discusses the diversity of deot in people and begins his legendary Hilchot De'ot by declaring, "Each and every person has many de'ot." He then lists many personality types: a grasping personality that is never satisfied, one who is satisfied with little, etc.
According to Maimonides (De'ot 1:4) one should always strive toward the de'ah habeinonit - the trait half-way between the two extremes. "Every person whose de'ot are in the middle is called a wise person."