A route specifically for bicycle traffic that runs alongside a road or street, separate from it. We prefer the term 'protected cycleway' - a cycleway that has some form of physical protection from motor traffic.
It should be clearly delineated from any adjacent footway, preferably by level and colour difference, and physically protected from the road - typically with a combination of kerbing, existing parking, and planting.
Not to be confused with cycle path - which is a distinct route for cycling, away from the road network.
The term "cycle track" is also a legally defined term in UK acts of parliament relating to creation of infrastructure, and the definition used is much broader than general usage:
Cycle Track means a way constituting or comprised in a highway, being a way over which the public have the following, but no other, rights of way, that is to say, a right of way on pedal cycles (other than pedal cycles which are motor vehicles within the meaning of the Road Traffic Act 1972) with or without a right of way on foot. [Section 329(1) Highways Act 1980; the words in brackets were inserted by section 1 of the Cycle Tracks Act 1984]. Cycle tracks may be created through conversion of a footway or footpath or newly constructed.
That is, the legal term "cycle tracks" emcompasses:
This usage is almost exclusively limited to legal and some highways engineering practice and, unless explicitly stated, is not what cycling campaigners mean when they say "cycle tracks".