When two vowels that are usually pronounced separately are pronounced together as a single sound.
It's a technical word used in linguistics and poetry.
Examples:
The word 'fire' has a synaeresis because the 'i' and 'e' are pronounced together.
The word 'cooperate' has a synaeresis because the 'o' and 'o' are pronounced together.
In the word 'quiet', the 'u' and 'i' make a synaeresis.
The word 'naive' has a synaeresis, but it's not easy to hear.
In the word 'naive', the 'a' and 'i' make a synaeresis, and it sounds funny.
The word 'poetry' has a synaeresis, and it makes the word sound different.