Serializability
With using concurrency, the term serializability is used to describe equivalence between outcomes.
The concurrent set of transactions are called transaction schedule or transaction history. The same set of transactions can be done concurrently, or in sequence.
If the outcome of the transactions is the same in either case, the transactions are concurrent transaction schedule is considered serializable.
Purpose
The goal of serializability is to keep the data in the data item in a consistent state, and is considered a strong criterion for the correctness of a concurrent transaction schedule.
Schedules that are not serializable are likely to generate erroneous outcomes.
The idea is that if one transaction is correct by itself (and order is irrelevant), then a schedule that consists of a sequence of these transactions is correct. Therefore, if a concurrent schedule is equivalent, then it is also correct.