5g would be the fifth generation of telecommunication.
An example of 5G is how a research paper might refer to a means of mobile communication 20 years in the future.
5G - Computer Definition
(2) The fifth generation of cellular service, superseding 4G LTE. Expected in the 2019-2020 time frame, 5G will increase transmission speed dramatically and embrace prioritization. As wireless data increases exponentially, real-time content such as video calling must be given a higher priority than Web pages. See cellular generations.