Proto-Indo-European Roots

Root/Stem: *ma'te'r-
Meanings:  a mother
Cognates (67):
Hellenic Greek me'te'r (a mother), Doric ma'te'r, New Greek metera
Italic Latin mater (a mother), Osc maatrei's, Umbr matrer;
Italian madre, Catalan mare, Spanish madre, Provencal maire, French mere, Brazilian mae, Portuguese mai
Celtic Common Celtic *ma'te'r, >
Gaulish matir (mother), Irish ma'thair (mother), Scottish ma'thair, Old Irish ma'thir, Welsh modryb (dame, aunt), Old Breton motrep (aunt), Breton moedreb (aunt)
Indic Sanskrit ma'ta' (a mother)
Dardic & Nuristani Kashmiri moju
Iranian Avestan ma'tar (a mother), Old Persian ma'dar
Ossetic mad, Baluchi math, Afghan mor, Tadzik modar, Persian madar, Waziri mor, mer
Tocharian Tocharic ma'car (a mother)
Armenian mair (a mother), gen. maur
Albanian motre" (a sister)
Germanic Common Germanic *mo'the'r, >
Old High German muoter, Old Icelandic modher, Old English mo'dor, Norse mo'thir, Old Low German mo'der, Old Frisian mo'ther, Old Saxon mo'dire (aunt), Middle High German mu"eder ,
Swedish mor, modor, German Mutter, Icelandic mooir, Norwegian mor, Danish moder, Faroese modir, Dutch & Afrikaans moeder
Baltic Common Baltic *mo'te' >
Lithuanian mote., motina (a woman), Latvian mate (a woman), Old Prussian mu'ti (a mother), pomatre (a step-mother), Sudovian ma'te' (a mother)
Slavic Common Slavic *mati (a mother), >
Ukrainian & Bulgarian & Serbo-Croatian & Slovene & Czech mati, Slovak & Russian mat', Belorussian & Polish & Upper Sorbian mac', Lower Sorbian mas' (a mother)
Notes: Again one of the most famous Indo-European words. Maybe Jakob Grimm and Rasmus Rask invented comparative linguistic staring at a table like this. 
Semantically this stem not everywhere denotes the mother - Albanian sister, Breton aunt, but as a whole the root is rather simple.
*Note: Old English and other ancient Germanic tongues used a special runes where we write th, dh here. Our th is like in thin, our dh reads like this.