An omission is something that has been left out, whether on purpose or by accident. If you're retelling a story to a friend but skip over an important detail, that missing part is an omission. When you fill out a form and accidentally leave a question blank, that's also an omission.

The word comes up frequently when people are comparing two versions of something to see what's different. A movie based on a book might have major omissions: whole characters or plot lines that didn't make it to the screen. A student writing a book report who forgets to mention the main conflict has made a significant omission.

Sometimes omissions are innocent mistakes. Other times, they're deliberate. If someone describes what happened during an argument but conveniently leaves out the part where they started it, that's a strategic omission. Lawyers pay close attention to omissions in contracts because what's not included can matter as much as what is.

The phrase sins of omission refers to wrong things that happen because someone failed to act or speak up when they should have. If you see someone drop their wallet but don't say anything, that's a sin of omission: you didn't do something bad, but you didn't do the right thing either. A better choice would be to speak up and help return it.