What is SOLID?

SOLID is an acronym for five basic principles of object oriented programming and design. Then applied properly these principles help to avoid big ball of mud and makes implemented system easer to extend and maintain.

S - Single responsibility principle;
O - Open/close principle;
L - Liskov substitution principle;
I - Interface segregation principle;
D - Dependency inversion principle.

For introduction to solid refer to Uncle Bob’s The S.O.L.I.D. Principles of OO and Agile Design talk at SkillsMatter.

For more information on SOLID principles I’d recommend to get the whole series of Uncle Bob’s videos from Clean Coders.