What are "Bulk" conditions in a semiconductor?
It simply refers to the part of the wafer/die that has not received additional implants or processing. Except when ... see below.

In the case of a transistor, it the well implant that is being referred to.  Again in this case it is the structure that the source/drain implants go into and is the background condition.

What might be confusing is that there is a "bulk" for the wafer and a "bulk" for the transistor with the transistor "bulk" actually being an implant into the wafer bulk through the well implants.

The common thread is that a "bulk" is therefore a background level of doping in a structure that is not that electrically active (i.e. you tend to have DC values applied - but not always).

It is considered to be more of a turn of phrase than a strict technical delimiter.