Θ - the letter named theta, generally transcribed as <th> in the Roman/Latin alphabet and representing the voiceless interdental fricative [θ] as in the initial sound of English thought (though in the earliest Greek the sound was apparently an aspirated voiceless alveolar stop like the initial sound of English take)
Ψ - the letter named psi, generally transcribed as <ps> in the Roman/Latin alphabet and representing an affricate, i.e. a combination of a stop, in this case [p], and a fricative, here [s]
Ω - the letter named omega, generally transcribed in the Roman/Latin alphabet as <o> or <o> with a macron or line over it (which I can't reproduce here due to the formatting restrictions of these WyzAnt answer text boxes) representing a tense rounded high back vowel--i.e., a "long o"--like the first syllable of English open
Φ - the letter named phi, generally transcribed as <ph> in the Roman/Latin alphabet and representing the voiceless labiodental fricative [f] as in the initial sound of English foot (though in the earliest Greek the sound was apparently an aspirated voiceless bilabial stop like the initial sound of English put)
So in the sequence that you give these letters, the equivalent in our alphabet would be "thpsoph," and as far as I know ΘΨΩΦ doesn't appear to be a word in Classical, Koine nor Modern Greek. Perhaps an acronym or abbreviation of some sort?