PostGIS shows a listing of [operators that make reference to `n-D`](http://postgis.net/docs/reference.html)

```
&&& — Returns TRUE if A's n-D bounding box intersects B's n-D bounding box.
&&&(gidx,geometry) — Returns TRUE if a n-D float precision bounding box (GIDX) intersects a geometry's (cached) n-D bounding box.
&&&(gidx,gidx) — Returns TRUE if two n-D float precision bounding boxes (GIDX) intersect each other.
<<->> — Returns the n-D distance between the centroids of A and B bounding boxes.
<<#>> — Returns the n-D distance between A and B bounding boxes.

```

What does "n-D" mean?