A pastiche is, according to The Concise Oxford Dictionary of Literary Terms:
a literary work composed from elements borrowed either from various other writers of from a particular earlier author. The term can be used in a derogatory sense to indicate a lack of originality, or more neutrally to refer to works that involve a deliberate and playful imitative tribute to other writers.
It could also be said in fewer words like Ben Szumskyj defined it:
a work of art that mixes style, mood, materials and elements of another artist.
The tales about Conan and Robert E. Howards other characters written solely by writers other than Robert E. Howard are generally referred to as pastiches – although they might not always meet the exact Oxford criteria above. The Howard Conan stories completed by L. Sprague de Camp and Lin Carter, and the non-Conan stories that they re-wrote as Conan stories, are not pastiches; they might be regarded as “posthumous collaborations”. Nor are adaptations of Howard’s stories in other media such as comics, movies and TV series. Although some comics stories, the 1980s movies, and the 1990s TV series clearly are pastiches.