What is Certain ?
Certain is (adj) 1. having no doubts about something Are you certain that you locked the door? I’m not certain where she lives. to make certain that to do something in order that something else will definitely happen He put the money in his safe to make certain that no one could steal it. 2. without any doubt Our team is certain to win the prize. 3. a certain quantity, a certain amount some The fire did a certain amount of damage. Rebuilding the house took a certain amount of time. You need to add a certain quantity of water to the paint. 4. which you do not know or are not definite about The manager is a certain Mr Arbuthnot. Certain fungi can make you ill if you eat them.
source: Easier English, Student Dictionary Upper Intermediate Level