every
Each one in a group, with none left out.
Every means each one in a group, without leaving anyone or anything out. When your teacher says “every student needs a pencil,” she means all students, not just some. When a sign says the library is open every day, it's open on Monday, Tuesday, Wednesday, and so on, with no days skipped.
The word emphasizes completeness and inclusion. If you check every answer on your homework, you've looked at all of them, not just the ones you thought might be wrong. If you invite every classmate to your birthday party, nobody gets left out.
Every works slightly differently than all. You'd say “every student” (singular) but “all students” (plural). The word focuses on each individual item or person in the group: every book on the shelf, every tree in the forest, every player on the team.
People sometimes use every for emphasis or exaggeration, like saying “I've looked everywhere!” when they mean they searched many places thoroughly. When something happens every single time, the speaker is stressing that there are no exceptions. The word helps us be precise about whether we mean some, most, or truly all.