What it is depends on who one talks to…scientists, theologians, philosophers.
Astrophysicist Hugh Ross offers 9 different possible definitions¹ for this complex word.
For more use the DOOR.
[“Strings” and “superstrings” involve looking at very, very tiny things that are smaller than the “bread-and-butter” protons, neutrons, and other atomic particles that run-of-the-mill relativity and gravity don’t, or can’t, explain well, and relate to quantum physics. There is much different thinking in physics in how to understand this, so we aren’t going to go into that here. We mention it only because in string-theory thinking at least 6 more dimensions–in addition to our familiar 4 (length, width, height, and time)–are commonly assumed to exist, making at least 10 dimensions altogether. Such fancy thinking is included in definitions 4 through 9 below.]
“…[S]cientists, theologians, and philosophers have different definitions of nothing depending on the context. It can mean a complete lack of:
2. matter and energy;
3. matter, energy, and the 3 big cosmic space dimensions (length, width, and height);
4. matter, energy, and all the cosmic space dimensions (including the 6 tiny space dimensions implied by string theories);
5. matter, energy, and all the cosmic space and time dimensions;
6. matter, energy, cosmic space and time dimensions, and created nonphysical entities;
7. matter, energy, cosmic space and time dimensions, created nonphysical entities, and other dimensions of space and time;
8. matter, energy, cosmic space and time dimensions, created nonphysical entities, and other dimensions or realms–spacial, temporal, or otherwise; or
9. anything and everything real, created, or otherwise.
“So what kind of nothingness did the universe come from? According to the space-time theorems of general relativity, not from the first five or possibly six on this list. In other words the universe could not possibly have arisen from matter, energy, and/or any of the space-time dimensions associated with them, either existing or previously existing. The reason number 6 [above] remains open to debate is that, depending on one’s theological/philosophical perspective, created nonphysical entities may or may not be endowed with the ability to create space-time dimensions.
“The space-time theorems also eliminate option number 9. The universe of matter, energy, space, and time is, in itself an effect. Every effect is generated by a cause. Absolute nothingness–the complete lack of anything and everything–cannot be a cause or causal agent…”
So the point here seems to be that if one looks for a Causal Agent, one has to look beyond here–beyond everything that has been teased out by scientific investigation.
¹ Everything cited here is from Hugh Ross’s Why the Universe Is the Way It Is (Baker Books, 2008, p.130). Color and boldface has been added to the original. Also, in several cases changing written numbers into actual numerals as we frequently do to make skimming and referring back easier.