Hi
Here are the Opinions of the some of the allinterview users 
posted in previous on Scalability Testing..

1.scalabilty is nothing but how many users that the 
application should handle


2.Scalability is nothing but maximum no of users that the 
system can handle


3.Scalability testing is a subtype of performance test 
where 
performance requirements for response time, throughput, 
and/or utilization are tested as load on the SUT is 
increased over time.


4.

as a part of scalability testing we test the expandability 
of the application.
in scalability we test 1.Applicaation scalability
                       2.Performance scalability
Application scalability: to test the possibility of 
implementing new features in the system or updating the 
existing features of the system. with the help of design 
doc we do this testing

Performance scalability:
 to test how the s/w perform when it is subjected to 
varying loads to measure and to evaluate the the 
performance behaviour and the ability fo the s/w to 
continue to function properly under different workloads.

--> to check the comfort level of an application in terms 
of user load. and user experience and system tolerance 
levels
--> the point within an application that when subjected to 
increasing workload begin to degrade in terms of end user 
experience and system tolerance
--> response time
    execution time 
    system resource utilization
    network delays
   
--> stress testing


Thanks & Regards
Suresh.Chaluvadi

Scalability is nothing but measurement, the metrics. it is 
mostly bound to the performance.

Scalability testing differs from  simple load testing in
that it focuses on the  performance of your Web sites,
hardware and software products, and internal applications at
all the stages from minimum to maximum load.

Scalability testing differs from  simple load testing in
that it focuses on the  performance of your Web sites,
hardware and software products, and internal applications at
all the stages from minimum to maximum load.