Originally Posted by Salahudeen
As Writer said above, you could simply quote the original definition and correctly reference it.
General tips for putting things into your own words:
1) Read the original. (e.g. the original says 'London bridge, as opposed to the Tower Bridge or the Millenium Bridge, is currently in the process of collapsing.')
it. (e.g. your thought process is 'basically, London bridge is falling down. Tower Bridge and the Millenium Bridge are irrelevant.')
3) Write based on your understanding rather than the original (e.g. you write 'London bridge is falling down.')
Obviously you need to adapt the writing style for academic writing and whatnot, but the key is to understand what the original is saying rather than simply changing a word here or there.