For an as-a-Service layer, elasticity is the degree to which the layer is able to adapt to workload changes by (de)provisioning services of its underlying layers in an autonomic manner such that at each point in time the utilised services fulfill the SLA of the layer as closely as possible.
Load is the characterisation of the quantity of requests.
Metric is a procedure to determine a quantity. SaaS quantity metric: e.g., #IaaS + #PaaS services IaaS quantity metric: e.g., #CPU services, #CPU Minutes, #CPU Invocations
Quantity is a property that can exist as a magnitude or multitude. [Wiki]
Example SaaS stakeholders: SaaS End-users, SaaS provider, SaaS Architects, SaaS Developers, SaaS Deployers (work on PaaS/IaaS layer!), SaaS Maintainer, SaaS *Anyone*
For an as-a-Service layer, scalability is the ability of the layer to sustain increasing workloads while fulfilling its SLA, potentially by exploiting a higher quantity of lower layer services.
For the Software-as-a-Service layer, scalability is the ability of the software to sustain increasing workloads while fulfilling its SLA, potentially by exploiting a higher quantity of PaaS or IaaS services.
Work is the characterisation of the data to be processed by a certain layer.
Workload is the combined characterisation of work and load.