The term "apt" has several meanings depending on the context:

1. As an adjective, "apt" means suitable, appropriate, or fitting for a particular situation. For example, an "apt comment" is one that is very relevant and to the point. It can also mean likely or prone to do something, such as "apt to make mistakes" meaning likely to make mistakes.
2. In computing and software, "APT" refers to the Advanced Package Tool, a package management system used by Debian and other Linux-based operating systems. It also can mean Advanced Persistent Threat, a type of stealthy and continuous cyber attack.
3. "Apt." is an abbreviation for "apartment."
4. In pop culture, "APT." is a song by Rosé and Bruno Mars, where the title is a romanized abbreviation of the Korean term for "apartment" and also refers to a popular Korean drinking game played in apartments.

Thus, "apt" as a common word usually means appropriate or likely, while "APT" as an acronym has multiple technical and cultural meanings. And "Apt." most commonly abbreviates "apartment." If you want the definition relevant to a specific context, please let me know! The most general meaning is suitable or likely.