Vanilla sex is a term that refers to what a society considers to be standard, traditional or normal sex. Different cultures, subcultures, and individuals have different ideas about what constitutes vanilla sex, so 'vanilla' has no direct meaning other than in relationship to the alternate under discussion. Often it is interpreted as sex that does not involve such elements as BDSM, kink, or fetish activities. Examples of the opposite of vanilla therefore could be fetish, 24/7 slavery, or edgeplay. Anything that is not vanilla might be described as kinky sex.
Among heterosexual couples in the Western world, vanilla sex often refers to the missionary position.
The term 'vanilla' derives from the use of vanilla extract as the most popular flavoring for ice cream, and by extension meaning 'plain' or 'conventional'.
Neapolitan is a term proposed to describe the sex life of a person that enjoys "vanilla sex" as well as other "flavours" of sex, such as BDSM. This is presumably coined in reference to Neapolitan ice cream, which has layers of different colours, including a layer of vanilla flavour, rather than being a reference to the inhabitants of Naples.
Similarly, vanilla clothing refers to conventional, non-fetish clothing. Its definition has varied over time; for example, much PVC clothing would now be deemed vanilla.