subject line
noun [ C, usually singular ] internet & telecoms specialized uk/ˈsʌb.dʒekt ˌlaɪn/ us/ˈsʌb.dʒekt ˌlaɪn/
（电子邮件的）主题（行）;主题，主题词
the space at the top of an email in which a line of text is written that tells you what the email is about, or the line of text itself
The email subject line was blank. 邮件主题行是空白的。
He thought of a great subject line for the next marketing campaign. 他为下一次营销活动想了一个很好的主题词。