Irrationalist is a wide term. It may be applied to mean one without rationality, for their beliefs or ideas. Or, more precisely, it may mean someone who rejects some aspect of rationalism, variously defined. For example religious faith may be seen as, in part, a rejection of complete rationalism about the world; this would be contested by some religious thinkers, in that the rational is a debatable term. On the other hand, it might be considered irrationalist to buy a lottery ticket, on the basis that the expected value is negative.