A person who is a non-latino caucasian.

In Mexico, a gringo is a US citizen. The word is not especially connotated (although one might claim that, the US being what it is and having done what it has done, said citizenship cannot be a neutral thing).

Gringos usually wear a convenient gringo uniform (it is convenient for everybody else, because it makes the gringos easier to recognize).

update: I am told that the word gringo in New Mexico is used with a negative connotation.

In order to protest the presence of American soliders (in their usual greens), Mexican civilians learned the English words, "Green go", as in "Green go home". "Green go" became Gringo - and the rest is history.

Grin"go (?), n. [Amer. Sp., fr. Sp. gringo gibberish; cf. griego Greek, F. grigou wretch.]

Among Spanish Americans, a foreigner, esp. an Englishman or American; -- often used as a term of reproach.

© Webster 1913