In Design ideation: the conceptual sketch in the digital age (2005) Ben Johnson defined ideation as "a matter of generating, developing and communicating ideas." His definition aligns pretty well with how I've been evangelizing and practicing the discipline. However, I place a much stronger emphasis on development and communication than on idea generation.
In most organizations, there is no shortage of ideas. Ideation helps you work through ideas quickly so you know where and how to invest.
Ideation is a process for bringing ideas to life. While ideation may include the creation of original ideas (generation), its primary focus is working through concepts (development) to gain insights, understand implications, gauge feasibility, and give teams a deeper understanding of potential product experiences (communication). You can think of ideation as a way of "building to think."
The output of an ideation effort needs to balance strategic initiatives with real customer needs to paint a complete picture of what it takes to turn an idea into a product or feature. Ideation deliverables can include presentations & strategic frameworks, product designs, and prototypes with actual data/users that communicate where an idea can go and why. These assets create experiences that help people understand the opportunity, scope, and limitations of ideas. Thereby enabling them to invest where it matters.