watashi

"Watashi" is a Japanese word corresponding to "I" in English. "Watashi" does not imply femininity; both sexes use it in ordinary conversation. "Boku" is used primarily by men and boys, but can sometimes be used by women. Much is often made by Americans and other foreigners of gender-specific speech and usage in Japanese, but much of this is overrated. Read _Japanese: The Spoken Language_ volume 1 (Noda) for more information.

watashi is actually a rather formal way to say "i" or "me" in Japanese. In familiar speech, men are more likely to use "o-re".

the female use of "watashi" in speech indicates a traditional societal prejudice towards a more proper behavior for women.

Since the definition Jeeves gives is already correct, I'll give the kanji for watashi:

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There are several forms of the word 'I' in Japanese.

Watakushi: The most formal way of saying I.
Watashi: Less formal than above.
Boku: Used in informal conversation. Usually used by males only, although some hip girls are using it nowadays.
Ore: Used in even more informal context. Again, males only.

Watashi is the word for 'I' first taught to people learning Japanese.

The difference in male and female speech is only in informal speech. It's like in English: A female using male speech is considered, uh, 'butch', and a male using female speech is considered 'a fruit'.