[gpm]

I think they're actually subtly different.

A bug is a computer program behaving unexpectedly. It can be caused by a defect. It can be caused by a bug crawling into the wiring and making the hardware malfunction. It can be caused by a cosmic ray flipping a bit.

A defect in a computer program is a place where the computer program that does not instruct the hardware to perform in a way so as to do what the program is intended to do.

Given the current (poor) state of our software, most bugs are defects, but not all. Sometimes it really is a bug shorting out a path on the motherboard causing the computer to not do what it was told to do.

Whether or not most defects are bugs is very case specific. Sometimes they're bugs. Sometimes they're just features that haven't been implemented yet, causing the computer program to not do what it is intended to do in an expected, rather than an unexpected, fashion.