Effect an act = do/cause/make
Commission عمل أو فعل
Omission امتناع عن فعل

الامتناع عن فعل قد يُجرَّم بنصٍ قانوني كما يرد في المثال التالي

Omission is not performing an act that is usually done or expected to be performed by a 'normal' person. LIke stopping at a scene of a traffic accident if you are a medical professional to assist, or not stopping if you are the first person, which is a crime. Commission is performing an act that results in some harm. Like, giving too much of a medication or too much oxgen or any other act that another. Think of it as omit vs commit. Unfortunately, from a legal standpoint, one is more liable of omission. Doing something unless it is blatantly wrong is not as liable.

Read more: http://wiki.answers.com/Q/What_is_an_act_of_omission_or_comm...