There are multiple definitions to visual literacy. Some of these definitions have evolved over time and some of the definitions are discipline specific. One well known and highly respected definition for visual literacy comes from the Association of College and Research Libraries. In the 2011 Visual Literacy Competency Standards for Higher Education, visual literacy is defined as,
"A set of abilities that enables an individual to effectively find, interpret, evaluate, use, and create images and visual media. Visual literacy skills equip a learner to understand and analyze the contextual, cultural, ethical, aesthetic, intellectual and technical components involved in the production and use of visual materials. A Visually literate individual is both a critical consumer of visual media and a competent contributor to a body of shared knowledge and culture".
There are seven standards listed in the 2011 ACRL Visual Literacy Standards. Below is a table that summarizes them. If you want to see the full list of standards, go to this link.