一人称代名詞. The many Japanese words that correspond to English "I" and "me" but differ in nuances such as formality, respect, intimacy, etc.
Japanese has many more words that correspond to English "I" and "me" that differ in nuances such as formality, respect, intimacy and other aspects, and all these words often shift in nuance over time.
Linguistically speaking, these words hardly satisfy any of the properties of pronouns in other languages such as English, and the only reason they are often treated as such is that they all translate to the first person pronoun in those languages. But when going the other way around, many (if not most) occurrences of "I" in English are not translated at in Japanese, leaving the sentence without a subject. That's because the real equivalent of the pronoun in Japanese, in its most common use (which is anaphora: referring to previously unmentioned information) is really the null-pronoun, i.e. leaving the subject (or any other part of the sentence) unspecified.
This feature of Japanese is called by generative linguists (who treat all languages as having pronouns in their "deep structure") pro-drop, that is: Pronoun-dropping.