If a word or syllable is unstressed, it is pronounced without emphasis.
[technical]
...the unstressed syllable of words like 'above', 'surround' or 'arrive'.
unstressed in British English
(ʌnˈstrɛst)
adjective
1.
carrying relatively little stress; unemphasized
2. phonetics
of, relating to, or denoting the weakest accent in a word or breath group, which in some languages, such as English or German, is also associated with a reduction in vowel quality to a centralized (i) or (a)
3. prosody
(of a syllable in verse) having no stress or accent