A reciprocal pronoun stands for a noun and indicates that two or more people are interacting by performing an action and receiving the results of the action. It is often translated one another.
A reciprocal pronoun indicates that two or more people are interacting with one another, both doing and receiving the action. In the NT, there is only one reciprocal pronoun, ἀλλήλων. It is always in the plural, and only in the accusative, genitive or dative. It never appears in the nominative or singular, so the lemma is the genitive plural.
καὶ ἔλεγον πρὸς ἀλλήλους (Mrk 4:41)
and they said to one another