There is no set definition for an act of war, cyber or otherwise.
An "act of war" and "armed conflict" are not defined within the Geneva Conventions. It is a matter of interpretation as to whether the threshold (within the context of surrounding circumstances) between a skirmish and a state of armed conflict has been crossed. Most cyber activity is criminal in nature. A key requirement to determining if there is an armed conflict taking place would be the deliberate intervention of members of a state's armed forces.
In determining if 'cyber' activity constitutes an armed attack, the UK view is that cyber-attacks can be considered as armed attacks if their consequences are essentially the same as those of a conventional kinetic attack. For example, theft of intellectual property would generally be considered to be a criminal act. However, if cyber-activity was performed on a scale which had sufficiently serious consequences for a developed economy, many States - including the UK - would likely regard this as an "armed attack".