the act of not including something or someone that should have been included, or something or someone that has not been included that should have been

Measures to control child employment are a glaring (= very obvious) omission from new legislation to protect children. 新的儿童保护法规显然遗漏了对雇用童工加以遏制的措施。

There are some serious errors and omissions in the book. 这本书里有几处严重的的错误和疏漏。

The fans believed that the omission of Heacock from the team was a serious mistake. 许多球迷认为球队不让希科克上场是一个很严重的错误。