Glossary
Email Subject Line
The headline of an email that determines whether recipients open it.
Definition
An email subject line is the first line of text recipients see in their inbox — the title of the email message. The subject line is the single most important factor determining whether an email is opened or ignored. Great subject lines are concise (30–50 characters to display fully on mobile), specific, curiosity-inducing, or urgency-driven.
Why It Matters
Email marketing consistently delivers the highest ROI of any digital marketing channel. But that ROI only materializes if emails are opened. The subject line determines the open rate, which determines click rates, which determines conversions.
Examples
Curiosity-driven'The writing mistake that's killing your CTR'
Benefit-driven'Write better headlines in 5 minutes — free checklist inside'
Urgency'Last chance: free guide expires tonight'
Weak (avoid)'Newsletter #47' — no hook, no value, no reason to open
Common Mistakes
Writing subject lines longer than 50 characters.
Using ALL CAPS or excessive punctuation.
Being vague.
Not A/B testing subject lines.