A simple question to answer, I thought, but so far I've been unable to find a reference that really clarifies it. The context for it is the Skill Expert feat in Tasha's Cauldron of Everything. Part of the feat description is given thus:
Choose one skill in which you have proficiency. You gain expertise with that skill, which means your proficiency bonus is doubled for any ability check you make with it. The skill you choose must be one that isn’t already benefiting from a feature, such as Expertise, that doubles your proficiency bonus.
I'm trying to determine whether that ‘one skill in which you have proficiency’ can include weapon proficiencies (eg. Martial Melee Weapons) or whether it is intended to be limited to things such as tool proficiencies, Acrobatics, and the like.
RAW and RAI answers equally welcome.
Best Answer
Weapon proficiencies are not skills
Skills are defined in chapter 7 of the Player's Handbook (and the Basic Rules). They are a specific set of proficiency which will let you add your proficiency on applicable ability checks. Tool proficiencies are another such source, albeit less used. You might note that the Skilled feat specifically refers to "proficiency in any combination of three skills or tools".
Weapon proficiency similarly allows you to add your proficiency to attack rolls made with the given weapon. But they are otherwise wholly independent and not the same as Skills. Note that there's a separate feat to Skilled (Weapon Master) for gaining proficiency with weapons.
To also cover the specific implication of the interaction asked about: The Skill Expert feat allows your to double your proficiency bonus for ability checks made with the chosen skill, so even if you were allowed to choose a weapon for this, it wouldn't apply to attack rolls (you know, what you usually use weapons for) and wouldn't give you a benefit until your DM called for eg. an Intelligence (longswords) roll.