Dream symbol: X
x, ex

Interpretation:
A kiss or a symbol of affection
Something unknown, unquantifiable, unspecified, the X factor

Marking something with X may indicate:
Something that is important
A beginning point
Something to watch out for
A choice

X may be a homonym for ex which could indicate:
Something former, from the past e.g. ex-boyfriend
Excluding or without
Outside of or out of

Related dream symbols: cross, kiss