364
It means type. size_t is the size type.
There were a few good explanations about the subject. Just to add another reason for re-defining the types:
In many embedded projects, all types are redefined to correctly state the given sizing to the types and to improve portability across different platforms (i.e hardware types compilers).
Another reason will be to make your code portable across different OSs and to avoid collisions with existing types in the OS that you are integrating in your code. For this, usually a unique (as possible) prefix is added.
Example:
typedef unsigned long dc_uint32_t;
If you're dealing with hardware interface code, the author of the code you're looking at might have defined int_t to be a specific size integer. The C standard doesn't assign a specific size to the int type (it depends on your compiler and target platform, potentially), and using a specific int_t type would avoid that portability problem.
This is a particularly important consideration for hardware interface code, which may be why you've first noticed the convention there.