What is an Acid?
An acid is any substance which produces H+ ions or H3O+ ions in water.
H+ ions are called hydrogen ions, H3O+ ions are called hydroxonium ions.

You will mostly see acids in reactions as forming H+ ions.
In reality, H+ is a single proton, and does not exist on its own.
It always attaches to something, in water it joins to H2O to form H3O+ ions.

This website will use either H+(aq) or H3O+(aq) to mean acid in water.