Like our English word day, it can mean a literal twenty-four hour period, the daylight portion thereof, or an indeterminate extended period, i.e., an "age". In fact, like most biblical Hebrew words, it is overloaded more than an English counterpart, due to a vast difference in the sizes of the vocabularies. Modern English has at least ten times as many words as biblical Hebrew.
The word yôm is used in Genesis 2:4:
This is the account of the heavens and the earth when they were created, in the day that the LORD God made earth and heaven. (Gen 2:4, NASB)
No matter how one views the "days" of creation—in this passage the sum-total of all God's creative efforts are summarized as "the day when the Lord God made earth and heaven " (the NIV simply translates "the day" as "when.) This verse is important in the debate because of its proximity to the creation account of Genesis 1.
There can be no doubt whatsoever: The interpretation of the creative days as twenty-four hour days is not demanded by the use of the word yôm in Genesis 1, no, not even by a literal interpretation. This is an important point: One does not have to interpret "day" as twenty-four hours in the Genesis account to claim literality. That would be the case if yôm exclusively meant a twenty-four hour period, but it is used frequently in the bible to refer to an unspecified duration. On this basis, purely discussing the issue of "days", the day-age theory can also claim literality1.
Other places where we find yôm, is in references to the "day" of God’s wrath. For example:
On the ground in the streets
Lie young and old;
My virgins and my young men
Have fallen by the sword.
You have slain them in the day of Your anger,
You have slaughtered, not sparing. (Lam 2:21)
The events described, the sacking of Jerusalem, destroying the walls, and taking the people captive occurred over a period longer than twenty-four hours.
Two of the Westminster Divines who are known to support twenty-four hour days of creation acknowledge other translations of yôm. John White wrote in his commentaries about its use in Genesis 2:4: "That is, in that Time that it pleased God to take up in forming them, which we know was in Six days, and not in One. But we find the Word, Day, in Scripture is used commonly to signifie Time Indefinitely."2 And John Ley in the Westminster Annotations, also on Gen. 2:4: "The day is not here taken (as in the first Chapter and in the beginning of this) for the seventh part of the week, but with more latitude for time in general wherein a thing is done, or to be done; as verse 17 & Luke 19.42. 2 Cor 6.2. Ruth 4.5."3
1 Although in general chronology is respected, the day-age theory requires overlap of the days if the life created was sustained by secondary (i.e., "natural") means from the time it was created by Divine fiat. For example, the vegetation (day three), if sustained by secondary means, required sunlight (day four) and insects and birds (day five). This is a violation of literality, while viewing "days" as "ages" is not. Some day-agers argue that during the period of creation, God sustained His creation actively (supernaturally) and so do not demand that the ages overlap.
2 John White, Commentary upon the Three First Chapters in Genesis (1656).
3 John Ley, Annotations upon All the Books of the Old and New Testaments (1645).