Is derived from the verb kṛ (8U) “to do” by addition of the suffix ‘man‘ to yield a noun karman meaning “action”. Technically, the term karman is called a substantive. In actual usage one must add an additional suffix specific to the type of use in a sentence. These suffixes added to substantives are equivalent to preposition one adds to noun such as by, for, from, of, on, in, at etc., in English. But in Sanskrit the suffixes are fused to the substantive to form what syntactically one calls a word. Thus karma is a word in the nominative case, first person singular of the neuter gender substantive karman. Since the word is so common in Enlgish language, I use the word karma in my blog posts.
The same word has another meaning as well and this meaning is what is normally meant by the word in English usage. This second, and the more popular meaning in widespread use both in Indic traditions as well as in English language is “result of action”, the correct word to be used in karmaphala, result or consequence of an action.
Technical: kṛ + manin (Uṇādi suffix) = kar + man + karman. The vowel of the verb takes a guṇa substitute, the bold letters i and n in the suffix are called code letters coding what changes the vowel of the verb takes during this addition, among other operations.