I grew up in only one family. But there were seven people in that one family. There was a Mom and Dad, and there were five children. Seven individuals, but only one family.
On the first day of Creation, God created everything necessary to sustain the earth, including the element of time. In Genesis 1:5 it says, “…And God called the light day, and the darkness He called night. And there was evening and there was morning, one day.” So, at the end of the first day’s creative work, the daytime and the nighttime were reckoned as one day. In other words, God established the 24-hour cycle of day and night that expresses the element of time. Literally, the text says, “And the evening and the morning – day one.”
The word “one” in the Hebrew is numeral one. But it has an interesting derivation. It comes from a verb that means “to unite something; to collect thing into a single unit.” So “one” in Hebrew really has the idea of collecting several things into a single unit, like one family or one bunch of grapes.
Well, on the first day of Creation, the collective unity was “the evening and the morning.” There were daylight hours and nighttime hours that made up one day. Now, because there are evening and morning parts to one day, it is difficult to figure out how one day could be millions of years, as “the day-age theory” purports. There would be no way to have a million years of light and then a million years of darkness. It is impossible to make sense of the two parts of the one day. One day, then, is a normal day of morning and evening.
Now you know the real meaning of the word.
Believers are another collective unity – we are one body in Christ.