dipthong

Two vowels that produce one sound. As in “aisle”. The “ai” is two letters but one sound. cf. improper diphthong (BBG 3.5).

Greek:

In Greek, the diphthongs are αι ει οι υι αυ ου ευ ηυ.

αι as in aisle αἴρω
ει as in eight εἰ
οι as in oil οἰκία
αυ as in sauerkraut αὐτός
ου as in soup οὐδέ
υι as in suite υἱός
ευ, ηυ as in feud εὐθύς or ηὔξανεν.
source
 

Hebrew:

See here.