A diphthong refers to the phenomenon where two vowels are pronounced consecutively within a single syllable.

My account is protected by 2-step authentication.

He is doing a special exercise to train his digastric muscle.

When learning Japanese grammar, it is important to understand the verbs of the second conjugation.

He made a model of a dodecahedron.

This house is warm even in winter because it has double-pane windows.

He defeated the enemy using a two-sword style.

I eat lunch at twelve o'clock.

This diode only allows current to flow in one direction.

My younger sister is twenty-one years old.