The official definition of "telework" can be found in the Telework Enhancement Act of 2010: "[t]he term 'telework' or 'teleworking' refers to a work flexibility arrangement under which an employee performs the duties and responsibilities of such employee's position, and other authorized activities, from an approved worksite other than the location from which the employee would otherwise work."
In practice, "telework" is a work arrangement that allows and employee to perform work, during any part of regular, paid hours, at an approved alternative worksite (e.g., home, telework center). This definition of telework includes what is generally referred to as remote work but does not include any part of work done while on official travel or mobile work.
You may also be familiar with the terms "telecommuting" and "flexible workplace" and both are sometimes used to describe what we now generally refer to as "telework." While "remote" and "mobile" work are also terms that are sometimes used as synonyms for telework, they tend to operate differently than telework as is apparent in the detailed operational definition.
For consistency, OPM recommends that all agencies use the term "telework" for reporting purposes and for all other activities related to policy and legislation, as defined in the Act.