A generation commonly refers to all persons who were born about the same time. (Ex 1:6; Mt 11:16) Associated with this is the meaning “contemporaries.” At Genesis 6:9 it is stated concerning Noah: “He proved himself faultless among his contemporaries [literally, generations].” When used with reference to family relationships, a generation can refer to a group of descendants, as sons and daughters or grandsons and granddaughters.​—Job 42:16.

The term can be used as a measure of time with reference to past or future ages. The generations of mankind descended from the sinner Adam have been transitory, as contrasted with the earth, which abides forever. (Ec 1:4; Ps 104:5) But the expressions “unnumbered generations” and “a thousand generations” refer to that which is to time indefinite. (1Ch 16:15; Isa 51:8) The command to the Jews that the celebration of the Passover was to be observed “throughout your generations” denoted continual performance to a time then indefinite. (Ex 12:14) God stated to Moses that Jehovah was his name as a memorial “to time indefinite,” “to generation after generation,” which implies forever. (Ex 3:15) The apostle Paul tells us that God is to be given glory “to all generations forever and ever.”​—Eph 3:21.

A generation may mean a class of persons, that is, those characterized by certain qualities or conditions. The Bible speaks of “the generation of the righteous one” (Ps 14:5; 24:6; 112:2) and “a generation crooked and twisted,” “a generation of perverseness.” (De 32:5, 20; Pr 30:11-14) Jesus Christ, when on earth, spoke similarly of the people of the Jewish nation of that day, and the apostle Paul applied such terms to the world of his day in general, which was alienated from God.​—Mt 12:39; 16:4; 17:17; Mr 8:38; Php 2:14, 15.

One Hebrew word for “generation” is dohr, corresponding to the Aramaic dar. (Da 4:3, 34) Dohr comes from a root verb meaning “stack in a circle” or “move around” (Eze 24:5; Ps 84:10) and thus has a basic underlying meaning of “circle.” The related word dur means “ball.” (Isa 22:18) The Greek equivalent is ge·ne·aʹ, from a root meaning “be born.”

Another Hebrew word, toh·le·dhohthʹ, is occasionally rendered “generations” or “genealogy” (Nu 3:1; Ru 4:18), also “descendants” or “families” (1Ch 5:7; 7:2, 4, 9) and “history” or “origins.”​—Ge 2:4; 5:1; 6:9; compare AS, AT, KJ, Dy, NW, RS, and other translations.

Length. When the term “generation” is used with reference to the people living at a particular time, the exact length of that time cannot be stated, except that the time would fall within reasonable limits. These limits would be determined  by the life span of the people of that time or of that population. The life span of the ten generations from Adam to Noah averaged more than 850 years each. (Ge 5:5-31; 9:29) But after Noah, man’s life span dropped off sharply. Abraham, for example, lived only 175 years. (Ge 25:7) Today, much as it was in the time of Moses, people living under favorable conditions may reach 70 or 80 years of age. Moses wrote: “In themselves the days of our years are seventy years; and if because of special mightiness they are eighty years, yet their insistence is on trouble and hurtful things; for it must quickly pass by, and away we fly.” (Ps 90:10) Some few may live longer, but Moses stated the general rule. Moses himself, who lived 120 years, was an exception, as were his brother Aaron (123 years), Joshua (110 years), and some others whose strength and vitality were unusual.​—De 34:7; Nu 33:39; Jos 24:29.

“This Generation” of Christ’s Prophecies. When Bible prophecy speaks of “this generation,” it is necessary to consider the context to determine what generation is meant. Jesus Christ, when denouncing the Jewish religious leaders, concluded by saying: “Truly I say to you, All these things will come upon this generation.” History recounts that about 37 years later (in 70 C.E.) that contemporary generation personally experienced the destruction of Jerusalem, as foretold.​—Mt 23:36.

Later that same day, Jesus again used practically the same words, saying: “Truly I say to you that this generation will by no means pass away until all these things occur.” (Mt 24:34) In this instance, Jesus was answering a question regarding the desolation of Jerusalem and its temple as well as regarding the sign of his presence and of the conclusion of the system of things. So his comment about “this generation” logically had an application down to 70 C.E. However, he was also using the word “generation” with reference to humans whose lives would in some way be associated with the foretold events during his presence.​—Mt 24.