Before we start any larger new product initiative, the responsible product person makes sure we have a written document of 1-3 pages that describes the initiative along seven sections:

A new initiative doesn’t come out of the blue, but has its genesis. This is described in the first section context. Two questions are answered.
Once the stage is set in this way, people are curious about the now what? This is answered by the next section.
Given the context, the responsible product person now has to answer the core question: What is it that we want to achieve? It is about intent, not a concrete solution. This is an important difference that is often missed. The intent has to formulated in such a way that – as Stephen Bungay says – the Spice Girls’ questions is truly answered: What do you really, really want? It must be clear enough that the team knows where to look for a solution. And it must be general enough such that the team has room to come up with a HOW to fulfill the intent.
For digital products, this means to state the job-to-be-done, the value proposition you want to achieve and the target group. As a little help, you can write this one sentence:
FOR [this user group] I want to PROVIDE [this value proposition] in order to GET [this job done].
It also helps to further explain why you can actually provide this value proposition, ideally even better than the competition. This triggers a discussion about the assets you have for a good solution.