Yesterday means the day before today. When you wake up on Tuesday morning, Monday was yesterday. When a new day starts at midnight, the day that just ended becomes yesterday.

The word helps us talk about the recent past. You might say “I finished my homework yesterday” or “Yesterday was my sister's birthday.” Unlike “last week” or “a few days ago,” yesterday is precise: it means exactly one day back.

Yesterday can also describe the recent past more generally, especially when things feel outdated. If someone talks about “yesterday's technology,” they mean older gadgets that seem ancient compared to what we have now. A history teacher might say that what feels like yesterday to them (perhaps the 1990s) seems like ancient history to their students.

One tricky thing about yesterday: it's always moving. What's yesterday today will be “two days ago” tomorrow. Yesterday is that one-day window into the past that follows you forward through time, always staying exactly one day behind.