What is the difference between "abduct" and "kidnap"?

What is the difference between abduct and kidnap?

In the main meaning, abduct and kidnap are rigorously synonymous. In addition to that, abduct also has a meaning in physiology (as in abductor muscle): move a body part away from its rest position.

Kidnap does have some slight suggestion of holding for ransom or for some other advantage over a targeted third party (originally of children to get money from their parents), and after this is delivered the victim will be released. Abduct is more general, and may include situations where the victims may be the targets themselves, without a third party being involved.

Sadly, this useful distinction is often lost.

Abduct and kidnap are synonyms. There may be legal definitions that vary, but essentially these are the same meanings and can be applied in the same way.