What is a online atomic mode backup

I am reading the SAP guide administrator of BO, and I am a little confused about some term they use regardless how to perform online or hot backups. Using the same words > For hot backups, use the database vendor's backup tools in online atomic mode It is the first time ever that I heard about online atomic backups. And what is the difference between these kind of backups and normal backups?? While the guide is talking about making offline backups they do not use the term **online atomic mode** so I think administrator should perform a normal backup (Normal for me) Could anyone make some light please?? Thanks in advance