základního
Základního is a grammatical form of the Czech adjective základní, which means basic, fundamental, or essential. It is the genitive singular form, used when the adjective modifies a masculine or neuter noun in the genitive case. In practice, you will encounter it in phrases such as základního tvaru (the base form) or základního významu (the fundamental meaning). The form is not a separate word with its own meaning; it is an inflected variant of základní that agrees with the noun it describes.
Základní comes from the noun základ, meaning foundation or base, with the common adjectival suffix -ní to
In ordinary Czech, the lemma or dictionary form of the adjective is základní. Základního is therefore primarily