The reference to 'happiness and bliss' is of particular importance as characterizing the positive nature of the state of consciousness attained in this first absorption (as well as in the next one, as we shall see shortly). These term, too, are used in highly specific technical senses in Buddhist meditation: happiness is the "contentedness at getting a desirable object' and bliss is the action experiencing of it when got. To Clarify this the Visuddhi Magga explains that if a man exhausted in a desert saw or learned about a pond on the edge of a wood, he would have happiness; but if he went into the wood's shade and used the water, he would have bliss.
Happiness plays a very important part already in the preliminary stages, while approaching and attaining the level of access to the absorption. In fact, when the meditator is endeavoring to develop concentration, happiness is both the positive fruit of his first deliberate, well-disciplined efforts and a motivation to further effort and progress. The manifestations of happiness vary in kind, degree and duration. Traditional terminology distinguishes the following: minor happiness, which is like a shiver 'raising the hairs on the body'; momentary happiness ' is like flashes of lightning at different moments; showering happiness breaks over the body again and again, like waves on the seashore; uplifting happiness is manifested not only as mental uplift, but also as producing a physical sensation of extreme lightness, as if one were floating on air; finally the pervading (or rapturous) happiness is when the whole body is completely pervaded, like a filled bladder, or like a rock cavern invaded by a huge inundation.
Amadeo Sole-Leris - Tranquility and Insight