1. Having three atoms; - said of certain elements or radicals 
2. Having a valence of three; trivalent; sometimes, in a specific sense, having three hydroxyl groups, whether acid or basic

Encyclopedia: Triatomic

• unilocular
• unimaginable
• unimaginably
• unimaginative
• unimaginatively
• unimagined
• unimitable
• unimodal
• unimpaired
• unimpassioned
• unimpeachable
• unimpeachably
• unimpeded
• unimportance
• unimportant
• unimposing
• unimpregnated
• unimpressed
• unimpressionable
• unimpressive
• unimpressively
• unimprisoned
• unimproved
• unincorporated
• unindustrialised