An act of displaying affection or love for another being.
While most animals kiss by touching each other with their noses, human beings (AKA people) are fairly unique in that they kiss by touching with their lips. For humans, there are various different protocols for kissing. To kiss another on the cheek indicates friendship and/or allegiance, and in some races it is an ancient ritual performed when greeting or parting. In a new culture it is advisable to carefully research kissing customs. Some people, for example, spit on each other's faces, while others (especially men greeting other males) prefer shaking hands, a good hard whack on the back or a pat on the bottom. Beware of varying these combinations.
Humans also kiss each other on the lips to share an intimate, passionate experience. If they stick their tongues in each other's mouths and engage in fighting with their tongues and sucking on each other's saliva it is known as a 'French kiss'. This is simply its given name, do not mistake it for the accepted form of greeting in France. Beware, the sexual act is likely to follow.
The general rule to follow is to kiss those you like, and if someone you don't like kisses you, wipe your hand over your lips and make lots of noisy spitting sounds.
Beware the 'kiss of death'. How do you know when you've received it? A good indication is that the person kissing you is a big, fat man called 'Don' who smells of cigars and grappa. If, while kissing you on the lips, Don squeezes your cheeks and sends 'the boys' out with you, you've had it - run like hell. If you don't, it is likely that death will follow.
Note: At Christmas time, elderly relatives smelling of cigars and cooking sherry take it upon themselves to deliver a surprisingly accurate simulation of the kiss of death. The difference here is that they send the dog out with you. While death is not imminent, it is still advisable to run.