Defined by many people!!!

Prof. of Computer Science: A kiss is a few bits of love compiled into a byte.
Prof. of Algebra: A kiss is two divided by nothing.
Prof. of Geometry: A kiss is the shortest distance between two straight lines.
Prof. of Physics: A kiss is the contraction of mouth due to the expansion of the heart.
Prof. of Chemistry: A kiss is the reaction of the interaction between two hearts.
Prof. of Zoology: A kiss is the interchange of unis*xual salivary bacteria.
Prof. of Physiology: A kiss is the juxtaposition of two orbicular ors muscles in the state of contraction.
Prof. of Dentistry: A kiss is infectious and antiseptic.
Prof. of Accountancy: A kiss is a credit because it is profitable when returned.
Prof. of Economics: A kiss is that thing for which the demand is higher than the supply.
Prof. of Statistics: A kiss is an event whose probability depends on the vital statistics of 36-24-36.
Prof. of Philosophy: A kiss is the persecution for the child, ecstasy for the youth and homage for the old.
Prof. of English: A kiss is a noun that is used as a conjunction; it is more common than proper; it is spoken in the plural and it is applicable to all.
Prof. of Engineering: Uh, What? I'm not familiar with that term