The gap theory is the:
"Hypothesis that there is a chronological gap between Genesis 1:2 and 1:3 during which geological evolution occurred. Or, alternatively, a period in which a race of souless 'men' lived." 
The gap theory maintains that an indefinite span of time exists between Genesis 1:1 and Genesis 1:2. This time span is usually considered to be quite large (millions of years) and is also supposed to encompass the so-called “geologic ages.” Proponents of the gap theory also suggest that a cataclysmic judgment came upon the earth during this gap period resulting from the fall of Lucifer (Satan). Therefore, they would say, the ensuing verses of Genesis chapter 1 describe a re-creation or reforming of the earth from a chaotic state and not an initial creative effort on the part of God.
Most proponents of the Gap theory claim a "Consecutive 24-hour day" creation (or re-creation), but place this after the Gap. This then may be termed an Old Earth view as opposed to the Young Earth view. This often leads to interpretational gymnastics to explain the creative events within each of the six days.
According to Tom Carpenter, there are many different versions of the gap theory but they all include these four concepts: (1) a literal view of Genesis, (2) a belief in an old age of the earth, (3) an opposition to evolution, and (4) a gap between Genesis 1:1 and Genesis 1:2 which is necessary to accommodate the geologic strata which, they believe, proves an old earth.