Tyga and Kylie Jenner -- Japanese Date Night

Tyga and Kylie Jenner were out together Monday night ... and we're told it was a date, but the way "date" is defined is, well, a sensitive issue.

The youngins went to Kabuki restaurant in Hollywood. Sources close to Tyga tell us they are "dating" but are quick to add that does not mean they are physical. It's important, because he's 24 and she's 17.

One Tyga source told us ... if we even insinuate they are having physical contact he will deny he's dating Kylie ... even though the source says Tyga privately says he's dating her.

BTW ... sitting on the same side of the booth is prima facie dating, right?