Quote Originally Posted by SoftPretzel
When тысяча is used by itself, it has a different spelling than when it is used with another number, such as десять тысяч.
тысяча - singular, nominative case
тысяч - plural, genitive case
Both are pronounced exactly the way they are spelled. If you "omit" 'a' in "тысяча" you'll effectively change the word's case and number.
In fast speech "a" in "тысяча" can be reduced, but it's still there.