The Outlander Series includes many words and phrases in Scottish Gaelic and Scots. The list will be expanded, but in the meantime, here's a list ( source ) of some of the common phrases:

Gaelic Edit

A leannan [a le-anan]: sweet-heart (vocative), with the implication of "baby" -- addressed to a daughter or other young person.

Ciamar a tha thu [Kia-mar a haa u]: greeting; "How are you?"

Mo chride [mo cree or mo kri-e]: my heart (used as a term of affection)

Mo nighean donn [mo nee-an down]: correct form of Jamie's term of endearment for Claire; My brown [haired] lass.

Mo charaid [mo kharaid]: my friend.
Clot-heid: clot-head or cloth-head; an idiot or imbecile.

Tha mi gle mhath [Ha Mi glay vah]: " I am well."

Scots Edit

Bairn/Wean: baby/child

Besom [BEE-zum]: a woman, generally ill-tempered.

Braw: literally, "brave," but also implies "fine, splendid, or excellent."

Dinna fash: Don't worry; don't be troubled or bothered.

Gomeral: fool, idiot.

Greet: to weep or grieve

Sassenach: outlander, or foreigner; more specifically an English person; usage generally derogatory.