Prefix used before a number or the question word 几 jǐ to form an ordinal number indicating sequence or rank

the third in a series; ordinal number three

Refers to the category used to distinguish between the speaker, the person being addressed, and others in a conversation, such as 第一人称 dì​yī​rén​chēng, 第二人称 dì'èr​rén​chēng, or 第三人称 dì​sān​rén​chēng