Another word from I’m OK – You’re OK.
–noun, plural –ses /-siz/
composition; constitution; makeup.
[Origin: 1595-1605; < Gk krâsis mixture, blend, equiv. to krā– (base of kerannýnai to mix) + –sis -sis]
n. 1. (Med.) A mixture of constituents, as of the blood; constitution; temperament.
2. (Gram.) A contraction of two vowels (as the final and initial vowels of united words) into one long vowel, or into a diphthong; synæresis; as, cogo for coago.
Source: The Free Dictionary.