The short line of text displayed as the title of an email in a recipient's inbox, shown before the email is opened, alongside the sender name and (often) a preview text snippet.
Why it matters
The subject line is usually the single biggest lever affecting open rate — it's the primary piece of information a recipient uses to decide whether an email is worth opening at all, which is why it's one of the most frequently A/B tested elements in email marketing.
In a sentence
"We test at least two subject line variants on every major send, since even small wording changes can meaningfully move open rate."
Related terms
- Often confused with: Preview text (see below).