Definition
A guideline is a piece of advice with authority behind it — a rule of thumb, standard, or principle meant to steer behavior without dictating every step. Health guidelines suggest how much to exercise; editorial guidelines shape how a magazine writes. Unlike a strict law, a guideline marks the recommended path and trusts you to follow it; it points the way rather than fencing you in, which is why guidelines are 'followed', 'issued', and sometimes quietly ignored.
Examples
Collocations
follow the guidelines·issue guidelines·official guidelines·in line with the guidelines·a rough guideline
Synonyms
rule·principle·directive·benchmark·standard
Antonyms
free-for-all
Word family
guide (verb)·guide (noun)
In TOEFL & IELTS
Common in TOEFL/IELTS texts on health, education, and the workplace. Usually plural ('the guidelines'). Collocations: follow/issue/set/breach guidelines. A guideline is softer than a 'regulation' or 'law' — advisory rather than binding — a useful distinction to draw in Writing. Note the spelling is one word.