The Oxford Dictionary of English defines optimism as "hopefulness and confidence about the future or the success of something".

Optimism bias is the demonstrated systematic tendency for people to be over-optimistic about the outcome of planned actions. This includes over-estimating the likelihood of positive events and under-estimating the likelihood of negative events. Along with the illusion of control and illusory superiority, it is one of the positive illusions to which people are generally susceptible. source: http://en.wikipedia.org/wiki/Optimism_bias

Defining quotes about optimism and over-optimism:

"Perhaps the best documented of all psychological errors is the tendency to be over-optimistic. People tend to exaggerate their own abilities. Like the children of Lake Woebegone, they are all above average. For instance, when asked if they thought they were good drivers, around 80% of people say yes! Ask a room full of students who thinks they will finish in the top 50% of the class, on average around 80% of them will respond in the affirmative — of course at least 30% of them will be disappointed at the end of the course"
Montier (2002)

"We all tend to be optimistic about the future. On the first day of my MBA class on decision-making at the University of Chicago, every single student expects to get an above-the-median grade, yet half are inevitably disappointed. This optimism will induce me to predict that economics will become more like I want it to be."
Thaler (2000)

"Overoptimism, according to which people in identifiable situations believe that the outcomes of events are better for them than for others. Thus, most persons underestimate the probability of being involved in an accident or contracting cancer or AIDS. Similarly, most people overestimate their capabilities. Thus, a large majority of motorists believe that they belong to the top 20 percent of drivers, which is, of course, objectively impossible. The same holds for their own evaluation of their work performance (Meyer 1975). The ipsative possibility set (the one they perceive as constraining their own actions) systematically deviates from the objective possibility set, which distorts behavior (Frey and Heggli 1999). Another case is the premarital expectation that one's own romantic marriage will be happy ever after and will not end up in divorce—an expectation that is quite unrealistic in view of divorce rates that are 50 percent and higher (Frey and Eichenberger 2001)."
Frey and Stutzer (2002)