There is no definition of ABH as ALL assault hold the same, which is something like
"The apprehension of immediate and unlawful personal violence"
Not to question Ethel, but there does not need to be any injury for a charge of ABH to be brought. The apprehension of violence is enough. That means if I pin you against a wall and punch your head really hard but miss, I can still be charged with ABH.
Such cases are best exampled buy GBH (one higher than ABH) in scenarios of "Psychological assault". The stated case, which I fail to remember, is a series of threats made over the phone of violence. This caused such pyschological trauma to the victim that a successful GBH was pursued and won. There was no injury to the victim.
Remember what one is arrested for, what one is charged for and what one is actually convicted for can be three different offences and often are in cases of assault. For example somebody can be arrested for ABH, say a bloody nose. Upon investigating it the person falls in to a coma and the suspect is arrested for GBH. When the victim awakes, it is discovered he or she often blacks out and always has bleeding noses. The assault was a mere push which came down to a mere common assault of battery. A hypothetical scenario yes, but similar cases often happens.
Upon until recently when you couldn't be arrested common assault, a lot of assaults were classified as ABH, particularly in domestic cases. Now with new arrest powers common assault is more prevelant.
Again, most ABH are not defined by the police or indeed medics, but by a CPS Lawyer, who has an official guideline called "Charging Standards". The charge is therefore simply made up on the CPS lawyers decision whilst reading the police officers statement.
If you say what the scenario is and injuries or lack of them present, I shall give you what I think the best charge would be.