Infinitesimal means incredibly, almost impossibly small. When something is infinitesimal, it's so tiny that it's nearly impossible to measure or detect, like a single grain of sand compared to all the beaches on Earth, or the chance of guessing a specific card in a shuffled deck on your first try.

Scientists use this word to describe things that exist but are almost too small to matter in practical terms. The amount of gold dissolved in seawater is infinitesimal: it's technically there, but you'd need to process millions of gallons to collect even a tiny speck. An infinitesimal change in temperature might be measurable by the most sensitive instruments but wouldn't be noticeable to a person.

Mathematicians especially love this word. They use it to describe quantities that approach zero but never quite reach it, which helps them understand how things change by the tiniest amounts. In everyday conversation, you might say your chances of winning the lottery are infinitesimal, or that you moved an infinitesimal amount closer to solving a difficult puzzle.