Japanese: 緊縛
Kin (緊): Tight
Baku (縛): as a noun, means something along the lines of “tying up; restraint; restriction; arrest​”.

Kinbaku means “tight binding” and usually translated as “the beauty of traditional shibari art”.
That is, bondage done in the traditional manner for an aesthetic / erotic effect. 
Another way to look at kinbaku is, the “process” of erotically tying and connecting with your partner.

Kinbaku is closely related to semenawa (“torture rope”), one hardly exists without the other.

The kanji 縛 is also used in shibari (縛り) but reads differently. There is much confusion among Westerners that shibari and kinbaku are the same (a.o. because of the same kanji used). I  have a blog on my site that explains more about the difference “Kinbaku vs Shibari“.

See also: Semenawa.