@adjective

Informally speaking, an adjective is a nounbase of a particular sort, that can have any of the three genders, and will take one or another depending on what non-adjective noun it links to. These nouns are descriptive by nature and can always be supposed to be describing another noun, even when that other noun does not appear in the sentence —

nRzaMso hanyAd etAm नृशंसो हन्यादेताम् "the cruel (guy) would kill her"

Example. We say —

kRSNo hayaH कृष्णो हयः "the stallion is black"

kRSNaM rakSaH कृष्णं रक्षः "the ogre is black"

kRSNA hayI कृष्णा हयी "the mare is black"

According to Sanskrit grammarians, the words for "black" here come from three different nounbases with the same letters kRSNa कृष्ण, and we must use the one that has the same gender as whatever it describes —

**kRSNa- कृष्णॱ m + /su → **kRSNas कृष्णस्

**kRSNa- कृष्णॱ n + /su → kRSNam कृष्णम्

**kRSNa- कृष्णॱ f + /su → **kRSNa- कृष्णॱ + /Ap + /su by ajAdyataSTAp → !**kRSNA कृष्णा

A noun that is not an adjective can also describe another noun, but it will not change gender when doing so, like the neuter rakSas रक्षस् ( ← **rakSas- रक्षस् + /su) here —

rakSas tvam रक्षस्त्वम् "you are an ogre"

Notice that strictly speaking kRSNA- कृष्णाॱ is not a nounbase, but a nounbase with /Ap added after it. I call kRSNA- कृष्णाॱ f a nounbase often in this website, that's a mistake, sorry.

How do you know that kRSNA कृष्णा is not a nounbase?

If it were, the GyAp ङ्याप् in sUtra GyApprAtipad... would be useless.

What's the word for adjective in the /pANinIya?

There isn't any. Not needed.