hebrew - What is the meaning of the term יום טוב (Yom Tov)?

I did a search in Tanac"h for the term יום טוב (along with any prefixes to the term)

The term appears a few times in Esther, esp. in the 2nd to last chapter. One example is:

Esther 9:19:

עַל־כֵּ֞ן הַיְּהוּדִ֣ים הפרוזים [הַפְּרָזִ֗ים] הַיֹּשְׁבִים֮ בְּעָרֵ֣י הַפְּרָזוֹת֒ עֹשִׂ֗ים אֵ֠ת י֣וֹם אַרְבָּעָ֤ה עָשָׂר֙ לְחֹ֣דֶשׁ אֲדָ֔ר שִׂמְחָ֥ה וּמִשְׁתֶּ֖ה וְי֣וֹם ט֑וֹב וּמִשְׁל֥וֹחַ מָנ֖וֹת אִ֥ישׁ לְרֵעֵֽהוּ׃

Therefore the Jews of the villages, that dwell in the unwalled towns, make the fourteenth day of the month Adar a day of gladness and feasting, and a good day, and of sending portions one to another.

I also found an example in I Samuel 25:8:

שְׁאַ֨ל אֶת־נְעָרֶ֜יךָ וְיַגִּ֣ידוּ לָ֗ךְ וְיִמְצְא֨וּ הַנְּעָרִ֥ים חֵן֙ בְּעֵינֶ֔יךָ כִּֽי־עַל־י֥וֹם ט֖וֹב בָּ֑נוּ תְּנָה־נָּ֗א אֵת֩ אֲשֶׁ֨ר תִּמְצָ֤א יָֽדְךָ֙ לַעֲבָדֶ֔יךָ וּלְבִנְךָ֖ לְדָוִֽד׃

Ask your young men, and they will tell you; wherefore let the young men find favour in your eyes; for we come on a good day; give, please, whatsoever comes to your hand, to your servants, and to your son David.’

Does the term יום טוב always imply a day of prohibition from work (prohibition against Melacha)? People wish each other "Good Yom Tov" on days such as Pesach, Shavu'ot and Sucot, but not during Hol Hamo'ed. I don't hear this expression used on Shabbat even though work is prohibited. On the other hand, on Purim, work is permitted, but I also don't hear the expression "Good Yom Tov" being used. And, for Purim, Esther calls it a Yom Tov, but people are working.

So, there seems to be a dichotomy. On the one hand, Shabbat which has a work prohibition is not called a יום טוב , but Purim which is called a יום טוב has work allowed. What's going on?

Also, does the term in Samuel also refer to a holiday of some type, or is this being used a general term meaning "a propitious day"?