There are many definitions for impact.
The National Institutes of Health provides a broad description of impact as "improving human health, fueling the U.S. economy, and creating jobs in our communities." For more information as to specific examples of impact resulting from NIH funding, see Impact of NIH Research.
The National Science Foundation uses a series of questions to illustrate the meaning of Broader Impacts:
“How well does the activity advance discovery and understanding while promoting teaching, training, and learning? How well does the proposed activity broaden the participation of underrepresented groups (e.g., gender, ethnicity, disability, geographic, etc.)? To what extent will it enhance the infrastructure for research and education, such as facilities, instrumentation, networks, and partnerships? Will the results be disseminated broadly to enhance scientific and technological understanding? What may be the benefits of the proposed activity to society?"
"Research Impact is a recorded or otherwise auditable occasion of influence from academic research on another actor or organization." Source: Maximizing the Impacts of Your Research: A Handbook for Social Scientists.
Research Councils UK (RCUK) defines research impact as "the demonstrable contribution that excellent research makes to society and the economy. Research impact embraces all the diverse ways that research-related skills benefit individuals, organisations and nations. These include:
"Research impact describes the effects and outcomes, in terms of value and benefit, as a result of research outputs." Source: Primary Health Care Research & Information Service. Flinders University, Adelaide, Australia.
“Research has a societal impact when auditable or recorded influence is achieved upon non-academic organisation(s) or actor(s) in a sector outside the university sector itself—for instance, by being used by one or more business corporations, government bodies, civil society organisations, media or specialist/professional media organisations or in public debate." Source:Wilsdon J, et al. (2015). The Metric Tide: Report of the Independent Review of the Role of Metrics in Research Assessment and Management.
Assessin the Impact of Research is a website that includes of list of research outputs and activities, and indicators that represent evidence of biomedical research impact.