Letter

信

xìn

(written message — see also missive)

信件

xìnjiàn

(written message — see also missive)

字

zì

(a symbol in an alphabet)

字母

zìmǔ

(a symbol in an alphabet)

尺牘 /尺牍

chǐdú

(written message — see also missive)

文字

wénzì

(a symbol in an alphabet)

書信 /书信

shūxìn

(written message — see also missive)