In the Christian religion, the Word of God, which is the second person of the Holy Trinity, JesusChrist. When used in this sense, "word" is a translation of the Greeklogos which more broadly means reason, principle, standard, or logic.
In computing, an integral data type that is usually equal to the size of a CPU's largest register or large enough to hold its largest addressable memory location. In the world of IBM-compatible PCs, "word" has become largely synonymous with "16-bit integer" due to the prevalence of early 16-bit x86 processors. Read on at computer word.
In hip hop slang, an exclamation, often not spoken in exclamatory voice, indicating deep and complete agreement. This use originates from the Five Percenters' mantra that "Word is Bond"; an earlier form was word up. The use of the term "word" as a sign of agreement and solidarity has come into common use outside of hip hop circles.