It is not redundant because the scalable does not have to be elastic. Not being able to remove resources is not an intrinsic feature of the scalable, even it can, and even if it can not or is not done, this is not necessarily a problem. Being scalable does not care about these things.
The problem is the definition of the term scalability. It is about the ability to meet larger demands, does not determine how, does not impose general requirements. A need for scale can impose specific requirements.
In addition, people understand scalability as the ability of servers to meet more demand. The term is much broader than this and can be applied in other contexts. We might be talking about software scalability .
The way to scale is not defined. One of these forms may be elasticity, it may be that what is climbing does not make sense to have elasticity.
On servers we can scale vertically or horizontally.
The ease and speed of making this scale change, whether to increase or decrease will determine if the system is elastic . Something elastic tends to use only the necessary resources. Need and scaling can even be managed by software. In general something is elastic if it can be done automatically.
The cloud is a mechanism and a form of marketing aimed at offering elasticity.
What most people do not realize is that being elastic does not mean spending less. It is very common to spend more. Much less being elastic is the best solution. If it was better and cheaper then everyone used it as well.
