Ok, sp1814 here's my definition. I don't know how closely it matches what's in the dictionary, but it's what I believe.
A racist is someone who believes that some races or ethnic groups are inherently inferior or of lesser worth than others.
If you hold those beliefs and never act on them you are still a racist. If you act on them by either verbal or physical assault, discrimination, preferential treatment, or simply expressing those opinions to others then you are actively racist, and there are laws in place to punish that behaviour. Quite rightly.
I also agree with what you said earlier, which is that harbouring a few racial prejudices is a long way from being a racist.
As an example of that I give you my mother.
She believes that all Italians have a hot temper, Germans can't be trusted, and that Jamaicans are 'laid back' like in the Bacardi advert.
That doesn't make her a racist. It makes her an ignorant old biddy who doesn't have a malicious bone in her body, and who is a product of the times she grew up in - ie non PC wartime Britain.
I think every single one of us 'buys in' to those kind of stereotypes to some extent, however small.