noun

• (Internet) A performance metric representing an instance where a post or ad is shown once.
  "Publishers are paid for each ad impression their site generates."
• (painting) The first coat of colour, such as the priming in house-painting etc.
• (engraving) A print on paper from a wood block, metal plate, etc.
• (philosophy) The vivid perception of something as it is experienced, in contrast to ideas or thoughts drawn from memory or the imagination.
  "Let us, therefore, use a little freedom, and call them Impressions; employing that word in a sense somewhat different from the usual. By the term impression, then, I mean all our more lively perceptions, when we hear, or see, or feel, or love, or hate, or desire, or will. And impressions are distinguished from ideas, which are the less lively perceptions, of which we are conscious, when we reflect on any of those sensations or movements above mentioned."