The Bible uses the term generation in some different ways. Normally, the word generation refers to all the people living at the same time—i.e., the word in the Bible has the same definition that we are used to in modern usage when we speak of Generation X or the Millennial Generation. Normally, a generation is about thirty years; one generation raises the next. However, in some biblical contexts, a “generation” can refer to a longer age or a group of people spanning a longer period of time.
In Genesis 2:4, “generations of the heavens and the earth” (ESV) seems to include all of human history—the era begun by the creation of the universe. In Exodus 1:6 the “generation” who dies refers to everyone who had been alive during the time that Joseph and his brothers lived. In Numbers 32:13, the “generation” is limited to Israelites—the group of them, twenty years old and older, at the time of their refusal to enter the Promised Land. That one generation was doomed to wander in the wilderness until they all died, except for Joshua and Caleb. When the plural word generations occurs in the Bible, as in Isaiah 51:9 and Acts 14:16, it refers to an indefinite period of time—many successive generations.
The original languages of the Bible used at least three different words that are translated “generation” in English. The Hebrew dor can refer to a normal, physical generation, as in Exodus 1:6. But it can also be used metaphorically to identify people of a distinguishable type. For example, Psalm 78:8 says, “They should not be like their fathers, a stubborn and rebellious generation, a generation whose heart was not steadfast, whose spirit was not faithful to God” (ESV). Here the word dor is used twice to refer to a group of people through a long period of time who were characterized by rebellion and sin. The “generation” in Psalm 78:8 is not limited to a normal thirty-year period but stretches back through the history of Israel to include all who were stubborn against God.
The other Hebrew word we translate as “generation” is toledot. This doesn’t refer to the character of a group or an age but to how that age was generated. So the “generations of the heavens and the earth” in Genesis 2:4 refers to the time periods that started with creation and continued organically from that point. The “generations of Adam” in Genesis 5:1 means the civilization of people that began with him. The next “generation” is that of Noah, to include the flood and the civilizations that came after. Shem’s influence is marked as a “generation” as he was the father of the Semites (Genesis 11:10). And Terah’s, because he left Ur with his son Abram (Genesis 11:27). Later, Ishmael (Genesis 25:12) and Isaac (Genesis 25:19) were the source of new generations. In each case, the men either experienced or caused a significant event that changed the course of their family line. They generated a culture-altering event.
In the New Testament, the Greek genea is the source of generation. It is similar to both Hebrew words. Literally, it means “fathered, birthed, nativity,” referring to a genetic line. But it can be used as both the time frame characterized by a specific cultural attitude and the people in that culture. In Matthew 1:17, the generations are marked off by significant events and people—Abraham, David, Babylonian captivity—like the Hebrew toledot. But when Jesus calls the Pharisees and scribes a “wicked and perverse generation,” He is referring to the culture that they lived in and encouraged (Matthew 12:39; see also Matthew 17:17 and Acts 2:40).
So, when we read “generation” in the Bible, we have to consider the context. Usually, a generation in the Bible is roughly thirty years long or the people living during that time, the same as what we understand a generation to be in everyday talk. But there are times when generation is used poetically to refer to a class of people demarked by something other than age.