邀请书的英文：
(a letter of) invitation

letter是什么意思：
n. 字母；信；字面意义；证书，许可证；文学
v. 用字母标明，写字母于；写印刷体字母；赢得校名缩写字母标志

in letter and in spirit
在字面意义和精神实质上|无论形式和实际

The letter was a forgery.
这封信是伪造的。

Post the letter through the slit in the letter box.
把这封信从信箱的投信口投进去。

Imprint a postmark on a letter =imprint a letter with a postmark
盖邮戳于信上

The letter is directed to him.
这封信是写给他的。

invitation是什么意思：
n. 邀请；请柬；引诱

Whomever you invite will be welcome.
你邀请的任何人都将受欢迎。

He was upset at not being invited.
人家没邀请他，他很不痛快.

A Bad padlock invites a picklock
坏锁招人撬

Entry to this competition is by invitation only.
此次竞赛的参赛只凭邀请函。

She was favored with an invitation.
她受到了邀请。