Vowel stems, in Indo-European linguistics, are the stems of nouns or verbs that are thematic. This means that the stems of these words end in a theme vowel that appears in, or at least influences, the inflections in the paradigms of these words.
In Latin, nouns of the first, second, fourth, and fifth declensions are considered vowel stems; the first declension has the theme vowel a, the second o, the fourth u, and the fifth e. Stems with i are treated together with consonant stems in the third declension. A similar situation appears in the Latin verb: the first conjugation contains vowel stems with a, the second with e, and the fourth with i. There are no Latin verbs with o or u, and there are very few Latin verbs that are athematic, which is where those verbs that are conjugated with consonant stems would be classified. Greek, by contrast, still preserves a large number of athematic, consonant-stem verbs.
Most other Indo-European languages have similar distinctions, or had them in their past. In the Germanic languages and the Celtic languages, the theme vowels are often hard to perceive because of the loss of final vowels; but their presence is still felt, in a manner that defines different ways of declining nouns or conjugating verbs; so philologists still occasionally speak of vowel stems and consonant stems in these languages also.