If someone chooses to quit college, I can refer to that person as a “former” student of that college. It therefore appears that I can use alumnus according to the definitions given for that term given by the References section given below.

However, that same condition also seems to match the definitions of a dropout.

Which one should I use here: is the former student an alumnus or a dropout?

References

1. alumnus

1.1. Oxford Dictionary of English on alumnus:

a male former pupil or student of a particular school, college, or university

1.2 Oxford English Dictionary on alumnus: ^{(paywalled links)}

Inflections: Plural alumni.
Origin: A borrowing from Latin. Etymon: Latin alumnus.
Etymology: < classical Latin alumnus foster-son, (in figurative use) son or child, ward, protégé, charge, pupil < alere to nourish (see ᴀʟɪᴍᴇɴᴛ n.) + an ablaut variant (zero-grade) of the same Indo-European base as ancient Greek ‑μενος, ending of medio-passive participles.

Compare ᴀʟᴜᴍɴᴀ n.

1. † A male pupil or student attending a particular school, university, or other seat of learning. Obsolete.

2. originally U.S.

   • a. A former pupil or student (typically male) of a particular school, university, etc.; a graduate of a particular seat of learning.

     ^{The singular alumnus almost always refers to a male, but the plural alumni is often used to refer to graduates of either sex.}

   • b. colloquial. In extended use: a former member of any group; an ex-member of a team, band, organization, etc.

2. drop-out, dropout

2.1 Oxford Dictionary of English on dropout:

a person who has abandoned a course of study or who has rejected conventional society to pursue an alternative lifestyle

2.2 Oxford English Dictionary on dropout: ^{(paywalled links)}

2. A person who ‘drops out’ (see to drop out 2 at ᴅʀᴏᴘ v. Phrasal verbs), esp. from a course of study or from society; also, the act of withdrawing. Also attributive. colloquial (originally U.S.).