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Fornication is sexual immorality BEFORE marriage.

The Greek word is "porneia" (πορνεία). Strong defines it as follows:

harlotry (including adultery and incest); figuratively idolatry: - fornication.

This means any sexual relationships "outside" of marriage. It can include relationships before marriage, or aldulterous relationships. The word "adultery" is usually translated from the Greek word "moicheuō" (μοιχεύω) or "moichos" (μοιχός). This word is almost exculsively used to describe the act of adultery, but "porneia" is a broader term that includes any sexual relationships outside of marriage.

Right. “Porneia” is a generic term indicating immoral sexual activity. Marriage is a covenant. Porneia breaks the covenant. (Alzheimer’s does not.)

I think the basic idea in porneia is prostitution (although it can be used of other sexual immorality). Porne was the ordinary word for a female prostitute.

In some ancient legal systems, a man who knew his wife was guilty of adultery was required to divorce her--otherwise it was thought that he and his wife were conspiring to blackmail the man or men she had sex with.

The Greek word is "porneia" (πορνεία). Strong defines it as follows: harlotry (including adultery and incest); figuratively idolatry: - fornication.

A dictionary does not say what a word means. It indicates what a word may mean (in different contexts, if you will). The bible translators chose "fornication" with great accuracy.

I speak English, so I looked "fornication" up in an English dictionary. "Fornication" means "sexual intercourse between a man and a woman not married to each other." As a mathematician, this is not altogether tight enough for me as a definition, so I will clarify.

fornication: both parties unmarried
adultery: any party married

In THIS CONTEXT (Matthew 19) [porneia] must mean "fornication" (sex before marriage) because otherwise the Bible would have used the word for "adultery" [moicheia].

The following is excerpted from The Bible, Marriage and Divorce

Christ and Divorce
When questioned by the major religious leaders of His day concerning the popular plan for divorce, the Lord Jesus Christ reaffirmed the Bible plan for marriage by citing Gen. 2:24 (Mt. 19:4-6; Mk. 10:11, 12; Lk. 16:18). The Pharisees attempted to force Christ to choose between the two popular plans for divorce, the Hillel position which was very liberal (divorce for anything) and Shammai position which was very conservative (divorce for infidelity). Christ did not concede to either position, but rather He cited the teaching given to Adam and Eve and declared "What therefore God hath joined together, let not man put asunder" (literally this verse means "stop severing what God has permanently put together") in Mt. 19:16. Furthermore, the Lord said "Whosoever shall put away his wife, except it be for fornication, and shall marry another, committeth adultery: and whoso marrieth her which is put away doth commit adultery" (Mt. 19:9).
This verse has already been interpreted by Matthew in his first chapter of the Gospel of Matthew (Mt. 1:18-25). The Jews had the custom of betrothal for approximately one year before the wedding. This meant that the Jewish man and woman entered into a betrothal covenant (Mal. 2:14) which could only be broken by divorce for unfaithfulness. The classic example of this is Joseph and Mary; Joseph thought Mary had committed fornication and had become pregnant and consequently he was going to divorce her ("put her away privily") during the betrothal period and before Joseph and Mary had become married through physical consummation. Although Joseph and Mary knew by divine revelation that Jesus was the virgin born Son of God, the Lord Jesus lived with the stigma of being a child of fornication (Jn. 8:41).
Christ's answer to the Pharisees was two-fold: marriage was for life and divorce was only for the betrothal period (cf. Lk. 16:18). The Bible allows for divorce only before physical consummation and not after physical consummation (i.e., not after marriage).

It can include ... adulterous relationships.

Can you supply an example of this usage from the NT?