What Are Requirement?

In engineering, a requirement is a singular documented physical and functional need that a particular product or service must be or perform.
It is most commonly used in a formal sense in systems engineering, software engineering, or enterprise engineering.
It is a statement that identifies a necessary attribute, capability, characteristic, or quality of a system for it to have value and utility to a user.
(http://en.wikipedia.org/wiki/Requirement)

In  own words "Requirement" means the need of getting through a path that should be followed by ones.A discribe in dictionary,
  : something required:
a : something wanted or needed : necessity <production was not sufficient to satisfy military requirements>
b : something essential to the existence or occurrence of something else : condition <failed to meet the school's requirements for graduation>

Functional requirements
describe the functionality that the system is to execute; for example, formatting some text or modulating a signal. They are sometimes known as capabilities.

DATA REQUIREMENTS
To identify and document the entities within the project scope and the preliminary data characteristics that will influence the definition of the technology infrastructure.

Environmental Requirements
inevitably dictate whether one particular "family" of worms will be suitable for culture in any given circumstance.

User characteristics
This covers the characteristics of users in three senses: the end user who will interact with the machine translation system; the end user of the final product of the translation process which may include for example, post-editing; the organisation deploying the machine translation system.


Usability Goals
Usability goals help focus attention and resources on the user and their issues. They challenge the design team to innovate and provide the basis for design tradeoffs. In addition to guiding design, usability goals are useful in customer interactions, evaluation, and testing.
Usability goals rely upon the development of realistic scenarios and add the dimensions of metrics and acceptance criteria.