Fundamentally means at the most basic, essential level, the core truth about something that doesn't change even when surface details do. If two things are fundamentally different, they're different in the most important ways, in their very nature or essence.

When someone says “I fundamentally disagree,” they mean they disagree with the core idea, not just minor details. If your teacher says that addition and multiplication are fundamentally connected (multiplication is repeated addition), she's pointing to the basic principle underneath both. When scientists discover something fundamentally new about how the universe works, they've changed our deepest understanding, not just added a small fact.

Think of a building's foundation: it's what everything else rests on. When something changes fundamentally, it's like rebuilding that foundation, so everything above it shifts too.

You might hear someone say that honesty is fundamentally important to friendship, meaning it's an essential requirement without which real friendship can't exist. Understanding what's fundamental about something helps you see what really matters versus what's just decoration or detail.