Tsaheylu is a Na'vi word meaning a bond (neural connection). This bond takes place when two creatures of Pandora connect their queues. Some instances where this bond would occur are those between a Na'vi and a direhorse, or a Na'vi and a mountain banshee. However, the bond can also occur between the Na'vi and other creatures like the thanator or the great leonopteryx. Almost every creature on Pandora has a queue used for tsaheylu, although some don't have any that are noticeable.
The Na'vi connect their queues during mating to create a strong, emotional, and lifelong bond.
The correct spelling of this word was controversial. In all official written sources, it was spelled tsahaylu prior to the DVD/Blu-ray release. Due to the pronunciation that is constantly used in the Avatar film, the correct transcription should be tsaheylu, because the -ay- sound coincides with the sound found in English words like eye, whereas the -ey- sound is present in words like they. This was confirmed by the DVD subtitles, where the tsaheylu spelling is used. Furthermore, the inventor of the Na'vi language, Paul Frommer, mentioned tsaheyl si as an expression for to bond with in an interview.
Before Paul Frommer was tasked with the development of a language for the Na'vi, James Cameron used shahaylu as the word for the bond as reflected by the script.