A stem is a contiguous segment string that consists of at least one °root and possibly some °affixes and that can be combined with an affix.
The definition is from Haspelmath (2025), and is similar to Aronoff & Fudeman's (2011: 2): "a stem is a base unit to which another morphological piece is attached". It implies that stems can contain inflectional affixes (not only derivational affixes), that only nouns, verbs and adjectives have stems, and that stems can contain stems.
"It might be clearer to say that a stem is something to which an inflectional affix can be added... That would mean that /book/ has a stem because -s can be added, while uninflected words such as /because/ do not have one." (Mugdan 2015: 259)