Carpe is the second-person singular present active imperative of carpō "pick or pluck" used by Horace to mean "enjoy, seize, use, make use of". Diem is the accusative of dies "day". A more literal translation of carpe diem would thus be "pluck the day [as it is ripe]"—that is, enjoy the moment. It has been argued by various authors that this interpretation is closer to Horace's original meaning 
Tu ne quaesieris, scire nefas, quem mihi, quem tibi
Ask not ('tis forbidden knowledge), what our destined term of years,
Perhaps the first written expression of the concept is the advice given by Siduri to Gilgamesh, telling him to forgo his mourning and embrace life although some scholars see it as simply urging Gilgamesh to abandon his mourning, "reversing the liminal rituals of mourning and returning to the normal and normative behaviors of Mesopotamian society."
In Horace, the phrase is part of the longer carpe diem, quam minimum credula postero, which is often translated as "Seize the day, put very little trust in tomorrow (the future)". The ode says that the future is unforeseen and that one should not leave to chance future happenings, but rather one should do all one can today to make one's own future better. This phrase is usually understood against Horace's Epicurean background. It has been argued that the meaning of carpe diem as used by Horace is not to ignore the future, but rather not to trust that everything is going to fall into place for you and taking action for the future today..
Collige, virgo, rosas ("gather, girl, the roses") appears at the end of the poem "De rosis nascentibus" ("Of growing roses", also called Idyllium de rosis) attributed to Ausonius or Virgil. It encourages youth to enjoy life before it is too late; compare "Gather ye rosebuds while ye may" from Robert Herrick's 1648 poem "To the Virgins, to Make Much of Time".
"De Brevitate Vitae" ("On the Shortness of Life"), often referred to as "Gaudeamus igitur", (Let us rejoice) is a popular academic commercium song, on taking joy in student life, with the knowledge that one will someday die. It is medieval Latin, dating to 1287.
Related but distinct is the expression memento mori (remember that you are mortal) which carries some of the same connotation as carpe diem. For Horace, mindfulness of our own mortality is key in making us realize the importance of the moment. "Remember that you are mortal, so seize the day." Over time the phrase memento mori also came to be associated with penitence, as suggested in many vanitas paintings. Today many listeners will take the two phrases as representing almost opposite approaches, with carpe diem urging us to savour life and memento mori urging us to resist its allure. This is not the original sense of the memento mori phrase as used by Horace.
In the 1989 American film Dead Poets Society, the English teacher John Keating, played by Robin Williams, famously says: "Carpe diem. Seize the day, boys. Make your lives extraordinary." Later, this line was voted as the 95th greatest movie quote by the American Film Institute.
Social philosopher Roman Krznaric suggested in his book, Carpe Diem Regained (2017), that carpe diem is the answer to consumer cultures schedules, timed work days, consumer culture and planning out our actions over the course of weeks and the weekends, instead of "just do it", with thought experiments for seizing the day rather than placing into calendars.