"In computer programming, SOLID (Single responsibility, Open-closed, Liskov substitution, Interface segregation and Dependency inversion) is a mnemonic acronym introduced by Michael Feathers for the "first five principles" identified by Robert C. Martin in the early 2000s that stands for five basic principles of object-oriented programming and design"
S.O.L.I.D introduced: Design Principles and Design Patterns, by Robert C. Martin, www.objectmentor.com (known as "Uncle Bob")
InfoQ presentation: SOLID Software and Design Patterns for Mere Mortals
Single responsibility principle
an object should have only a single responsibility.
“software entities … should be open for extension, but closed for modification”.
Liskov substitution principle
“objects in a program should be replaceable with instances of their subtypes without altering the correctness of that program”. See also design by contract.
Interface segregation principle
“many client specific interfaces are better than one general purpose interface.”
Dependency inversion principle
one should “Depend upon Abstractions. Do not depend upon concretions.”
Dependency injection is one method of following this principle.