While "the cloud" has largely become marketing jargon, there is an actual widely-accepted definition, with real requirements to qualify as being considered "in the cloud." The NIST (National Institute of Standards and Technology) in the US has drafted the most widely-accepted definition, which as been adopted by the Cloud Security Alliance:
The NIST Definition of Cloud Computing
Cloud computing is a model for enabling ubiquitous, convenient, on-demand network access to a shared
pool of configurable computing resources (e.g., networks, servers, storage, applications, and services) that
can be rapidly provisioned and released with minimal management effort or service provider interaction.
This cloud model promotes availability and is composed of five essential characteristics, three service
models, and four deployment models.
On-demand self-service. A consumer can unilaterally provision computing capabilities, such as
server time and network storage, as needed automatically without requiring human
interaction with each service’s provider.
Broad network access. Capabilities are available over the network and accessed through standard
mechanisms that promote use by heterogeneous thin or thick client platforms (e.g.,
mobile phones, laptops, and PDAs).
Resource pooling. The provider’s computing resources are pooled to serve multiple consumers
using a multi-tenant model, with different physical and virtual resources dynamically
assigned and reassigned according to consumer demand. There is a sense of location
independence in that the customer generally has no control or knowledge over the exact
location of the provided resources but may be able to specify location at a higher level of
abstraction (e.g., country, state, or datacenter). Examples of resources include storage,
processing, memory, network bandwidth, and virtual machines.
Rapid elasticity. Capabilities can be rapidly and elastically provisioned, in some cases
automatically, to quickly scale out, and rapidly released to quickly scale in. To the
consumer, the capabilities available for provisioning often appear to be unlimited and can
be purchased in any quantity at any time.
Measured Service. Cloud systems automatically control and optimize resource use by leveraging
a metering capability1 at some level of abstraction appropriate to the type of service (e.g.,
storage, processing, bandwidth, and active user accounts). Resource usage can be
monitored, controlled, and reported, providing transparency for both the provider and
consumer of the utilized service.
Cloud Software as a Service (SaaS). The capability provided to the consumer is to use the
provider’s applications running on a cloud infrastructure. The applications are accessible
from various client devices through a thin client interface such as a web browser (e.g.,
web-based email). The consumer does not manage or control the underlying cloud
infrastructure including network, servers, operating systems, storage, or even individual
application capabilities, with the possible exception of limited user-specific application
Cloud Platform as a Service (PaaS). The capability provided to the consumer is to deploy onto
the cloud infrastructure consumer-created or acquired applications created using
programming languages and tools supported by the provider. The consumer does not
manage or control the underlying cloud infrastructure including network, servers,
operating systems, or storage, but has control over the deployed applications and possibly
application hosting environment configurations.
Cloud Infrastructure as a Service (IaaS). The capability provided to the consumer is to provision
processing, storage, networks, and other fundamental computing resources where the
consumer is able to deploy and run arbitrary software, which can include operating
systems and applications. The consumer does not manage or control the underlying cloud
infrastructure but has control over operating systems, storage, deployed applications, and
possibly limited control of select networking components (e.g., host firewalls).
Private cloud. The cloud infrastructure is operated solely for an organization. It may be managed
by the organization or a third party and may exist on premise or off premise.
Community cloud. The cloud infrastructure is shared by several organizations and supports a
specific community that has shared concerns (e.g., mission, security requirements, policy,
and compliance considerations). It may be managed by the organizations or a third party
and may exist on premise or off premise.
Public cloud. The cloud infrastructure is made available to the general public or a large industry
group and is owned by an organization selling cloud services.
Hybrid cloud. The cloud infrastructure is a composition of two or more clouds (private,
community, or public) that remain unique entities but are bound together by standardized
or proprietary technology that enables data and application portability (e.g., cloud
bursting for load balancing between clouds).
The NIST Definition of Cloud Computing (Draft)
CSA: Cloud Computing Architectural Framework