Full Idea
'Impressions' are our more lively perceptions, when we hear, see, feel, love, hate, desire or will. 'Ideas' are less lively perceptions, when we reflect on any of those sensations or movements.
Gist of Idea
Impressions are our livelier perceptions, Ideas the less lively ones
Source
David Hume (Enquiry Conc Human Understanding [1748], II.12)
Book Reference
Hume,David: 'Enquiries Conc. Human Understanding, Morals', ed/tr. Selby-Bigge/Nidditch [OUP 1975], p.18