A specific combination of operating system and hardware. Some platforms are:

Linux on Intel processor ("normal" PC hardware)
Windows98. We don't have to worry about hardware
Sun's UNIX OS on the UltraSPARC processor
Linux on a DEC Alpha processor.
FreeBSD on Intel

And the list could go on and on....

As you can see though, it is not always enough to specify the OS or the hardware to define a platform. Code compiled on Linux/i386 won't work so hot on Linux/Alpha, or Win98. Now it might work on FreeBSD/i386 or something, but that is because they are being tricky...