indefinable /in-di-fīˈnə-bl/

1. That cannot be defined
2. Hard to identify or describe

• letter-carriers
• letter-clip
• letter-clips
• lettered
• letterer
• letterers
• letter-founder
• letter-founders
• letter-gae
• letterhead
• letter-heading
• letterheads
• lettering
• letterings
• letterless
• letter missive
• lettern
• letter of attorney
• letter of comfort
• letter of credit
• letter of indication
• letter-of-marque
• letter of peace
• letter-perfect
• letterpress