A wetland that accumulates peat.
Fen is a tricky concept. Why? Because there is no strict definition of what it is that separates a fen from a bog. All sources agree that a fen is less acidic than a bog. Some claim that a fen is alkaline or neutral; others claim that a fen may have any pH above five (moderately acidic and on into alkaline).
In most cases, the difference in acidity is due to the source of the water. A fen generally gets the majority of its water supply from groundwater, while a bog gets the majority of its water from the rain. Rainwater is acidic, containing carbonic acid; groundwater is full of minerals that have reacted with the carbonic acid, reducing the acidity.
We start with a fen. It is a piece of low ground that drops below the water table, becoming marshy (it may even start out as a pond). It starts to form a layer of peat (decomposing plant material) which slowly fills the fen; once the layer of peat becomes so thick that the roots of the plants growing in the fen can't reach the mineral-rich groundwater, the ecosystem undergoes a shift. While many species (Heathers, Sundews and Deer Sedge, for example) start to move in to take the place of the malnutritioned plants, the key species is bog moss, AKA Sphagnum.
Sphagnum makes the ground more acidic, which partially accounts for the acidity of bogs. Sphagnum also acts as a wick for the water, drawing it up above the water table -- in fact raising the water table up above what it would otherwise be. It also stores rainwater, keeping the ground wet and spongy during dry periods. Although bogs may be 85% to 98% water, they are solid enough to walk on.
So we have a semi-solid, acidic, mineral-poor bog, as opposed to a marshy, alkaline, mineral-rich fen. We may not be able to say exactly when the change occurs, but we can see that it does.