A linguistic concept. Syntax is the way how words are combined to phrases or sentences. Syntax is commonly defined as a set of rules, which, if followed, are capable to produce all possible sentences (or programms if you consider computer languages as well) of a specific language correctly.
There are psychologists that assume that there has to be a syntax in our brains to understand language at all.
One (possible) rule of the english language: "In declarative sentences the word order is subject, predicate, object", e.g. Tom (subject) is sitting (predicate) on the chair (object).