subject line
US /ˈsʌbdʒekt laɪn/
UK /ˈsʌbdʒekt laɪn/

1.
主题行, 邮件主题
the line of text that appears in an email, indicating the topic of the message
:
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Please make sure to include a clear subject line in your email.
请务必在您的电子邮件中包含清晰的主题行。
•
A good subject line can increase the open rate of your emails.
一个好的主题行可以提高您电子邮件的打开率。