cloud computing defined

a formal definition:
Cloud Computing | NIST
Recommendations of the National Institute of Standards and Technology

NIST SP 800-145, The NIST Definition of Cloud Computing (PDF)
Essential Characteristics:

• On-demand self-service
• Broad network access
• Resource pooling. 
• Rapid elasticity
• Measured service.

Service Models:

• Software as a Service (SaaS)
• Platform as a Service (PaaS).
• Infrastructure as a Service (IaaS)

Deployment Models:

• Private cloud
• Community cloud.
• Public cloud.
• Hybrid cloud.