Although there is a definition of bisexuality (as a sexual orientation) given in the dictionary, the use of the word bisexual as a label and identity varies from group to group and from bisexual individual to bisexual individual.
Since no one definition can fully cover all the different types of bisexuals that exist in this world, here are a few of the more popular definitions currently in use:
1. Someone who is capable of feeling romantic, spiritual, and/or sexual attraction for two genders*.
2. A person who loves despite gender.
3. One who loves individuals first and genders second.
4. An individual open to sexual or emotional exploration with two genders.
*note that two genders is specified here being as many people are aware that there are more that two genders in this world (gender refers to a person's psychology, not to their physical sex)