Self-Directed Learning is a process in which learners take full ownership and responsibility of learning. In simple words,
- Why learn
- What to learn
- How to learn
- How much to learn and so on.
There are similar words that are frequently used in this context. Some of these words are self-study, self-learning, autodidact, self-education, self-taught.
Below mentioned are some of the references where self-directed learning or associated word is used.
“In classical antiquity, Self-study played an important part in the lives of such Greek philosophers as Socrates, Plato, and Aristotle. Some historical SDL included Alexander the Great, Caesar, and Descartes. Social conditions in Colonial America and a corresponding lack of formal educational institutions so many people have to learn on their own (Hiemstra, 1994)” 
“Breaking new ground in Washington scholarship and American revolutionary history, Adrienne M. Harrison investigates the first president’s dedicated process of self-directed learning through reading, a facet of his character and leadership long neglected by historians and biographers. In A Powerful Mind, Harrison shows that Washington rose to meet these trials through a committed campaign of highly focused reading, educating himself on exactly what he needed to do and how best to do it. In contrast to other famous figures of the revolution—Thomas Jefferson, John Adams, Benjamin Franklin—Washington did not relish learning for its own sake, viewing self-education instead as a tool for shaping himself into the person he wanted to be. His two highest-profile and highest-risk endeavors—commander in chief of the Continental Army and president of the fledgling United States—are a testament to the success of his strategy” – The Powerful Mind, author Adrienne M. Harrison talks about the self-education of George Washington 
As Diane explains in her book [Prepared], Summit’s unique model [Summit Public Schools] is built on three key elements. And one of the keys is Self-Directed Learning. Self-directed learning: With the support of their teachers, all students are responsible for setting their own learning goals, developing learning plans, testing their knowledge, and assessing their performance. The personalized learning approach allows students to learn at their own pace. This is an incredibly important skill that will benefit them throughout their lives. Prepared – Book Review Bill Gates