In one short sentence: it's a set of processes and tools that helps DEV and OPS teams work faster.
For me, devops is a culture, it's a way of thinking that both DEV and OPS teams has to adopt in order to work well.
The most important requirement is clear and transparent communications between the teams (it can begin with the definition of a common vocabulary, the required documentations for a software, logging and security policies...)
Then, using automation, implement tools to help the teams increase their productivity and work faster (continuous build for DEV, PAAS for DEV, simple delivery pipelines for OPS...).