happening by chance

chance, random

happening unexpectedly

accidental, unforeseeable

having a strong possibility of being unsuccessful

dangerous, risky

dependent on something that may or may not happen

conditional; possible

happening by happy chance

lucky, accidental

(of a number) not exactly divisible to two: '1, 3, 5 and 7 are odd numbers'; one of a pair when the other is missing; not regular, habitual or fixed; strange; peculiar

miscellaneous, various

without any particular reason

haphazard, chance

not foreseeable

unpredictable

not predictable

changeable