Question

θ English bath, thin, ether. Note that English th can also spell the voiced version of this sound [ð], as in these, other. Most dialects of Spanish don't have this sound, but Castillian Spanish does: it is spelled z or c (before e or i): zapato, cero, circo.

Accepted Answer

Voiceless Dental Fricative http://en.wikipedia.org/wiki/Voiceless_dental_fricative

Symbol	θ
Name	Voiceless Dental Fricative
Notes	English bath, thin, ether. Note that English th can also spell the voiced version of this sound [ð], as in these, other. Most dialects of Spanish don't have this sound, but Castillian Spanish does: it is spelled z or c (before e or i): zapato, cero, circo.
Ref	http://en.wikipedia.org/wiki/Voiceless_dental_fricative

C Dental Fricative English E Er Spanish Z