I think the fact that you're hiding your escapade from your wife and are, yourself, wondering if you cheated speaks volumes. If, in fact, you felt comfortable enough to relieve yourself of your guilt by being upfront with your wife, then your internal struggle would be solved. You came here to see how many would 'be in your corner' and then use that information as some sort of 'justification' when telling your wife. But being there are a number of members telling you 'it was cheating' has now totally defeated the whole purpose of posting.
Yes, cheating is: anything you have to hide, lie, or sneak to do. If you have to resort to any of these behaviors then you are cheating on your SP, period.
You need to love/respect your wife enough to tell her the truth and deal with whatever consequences there may be. Otherwise the guilt you're feeling will eat you from the inside out. Whether you like it or not it will affect your marriage.