David Cameron has said he will fight with everything he has to avoid having to go into coalition. However, according to a recent London School of Economics (LSE) analysis report, 9% is the current estimated likelihood of one party winning an overall majority.
A coalition government is a cabinet of a parliamentary government in which several political parties cooperate, reducing the dominance of any one party within that coalition. The usual reason given for this arrangement is that no party on its own can achieve a majority in the parliament. Wikipedia