The pasuk that speaks about a Sotah being unfaithful says that she committed meilah against her husband (bamidbar 5:12). I am only familiar with that term being used in connection with stealing or using something that belongs to God or the temple. Most translations I have seen explain it as something like breaking faith. Why is the term meilah used by a sotah?
Although in the Mishna Me'ilah is of course most associated with using something from the Temple (cf. Leviticus 5:15), in the Bible it is used in the context of multiple sorts of other transgressions. For example, besides for this example of a woman cheating on her husband, it is used in reference to monetary crimes against regular people (Leviticus 5:21), taking the forbidden spoils of Yerikho (Joshua 7:1).[i] It is also used for sin more broadly (cf. Leviticus 26:40, Ezekiel 39:23) and idolatry in particular (I Chronicles 5:25).
So what does the word mean?
Metsudat Tsion (Ezra 9:2) explains it to mean sin, and Rabbenu Bahya (Leviticus 5:15) explains that it means a deliberate transgression in particular. These fit in the context of the sotah, as does your proposed definition of "breaking faith". Unlike a husband who can marry multiple wives, a wife is limited to a single husband, and thus effectively violating her contract with her husband through relations with another man.
Alternatively, Panim Yafot (there) suggests that she sins against her husband by bringing punishment to him in accordance with Sotah (4b) which states that "immorality in the house brings poverty to the house".
[i] According to the peshat, this wasn't like the Temple or its vessels; just forbidden (Cf. Radak to 6:17).
Netziv in HaEmek Davar explains the terms connotes a complete exit from the husbands domain as the husband is now forbidden to have marital relations with her.
As noted in another answer Ish can also refer to God (the word Ish is in fact used twice in that verse) and according to the Zohar (3:123a) there is an aspect of sinning against God as well, hence the sotah ordeal takes place in the temple and involves the erasure of God's name which (homiletically as per bKiddushin) is infused in the matrimony of a man and a woman(cf Bamidbar Rabba 9:1, Likutei Halachot Even HaEzer, Gittin 4:5).
The gemarah in Meilah 18a-b states
כי תמעל מעל אין מעל אלא שנוי וכן הוא אומר (במדבר ה, יב) איש איש כי תשטה אשתו ומעלה בו מעל ואומר (דברי הימים א ה, כה) וימעלו באלהי אבותיהם ויזנו אחרי הבעלים
[The term] ma'al denotes nothing else but [effecting] a change, and thus it says. If any one's wife go aside and act unfaithfully [ma'al] against him . . ., and it also says, And they broke faith [wa-yim'alu] with the God of their fathers. and went astray after the gods of the peoples of the land. (Source, p.40)
The term meilah implies a change of affiliation. As Rashi there explains
וכן הוא אומר וימעלו וגו' ויזנו אחרי הבעלים - ששינו עצמם מן המקום לעבודת כוכבים:
Tosfot further clarifies that her change is a breakdown in the sanctity of the marriage
ומייתי ראיה מסוטה שהיא עושה שינוי שמנחת בעלה ועוסקת ומדבקת בדבר שהוא חולין וגנאי לה
@Mevaqesh posted an excellent answer - I'm just posting this to clarify the points made by commenters, as well as to add a fantastic source to this discussion, which Mevaqesh reminded me of.
The contrast between the Meilah you refer to and the Meilah in a Sotah is based on who the Meilah is against (we can use the accepted definition of taking or of sinning for the purposes of this discussion). In the case of sinning against Hashem (like taking things from the Beis Hamikdash), it is ומעל מעל בהשם, in cases of plain sinning against another person (like taking money from them) is is just מעל, and in the case of Sotah, as noted by @DoubleAA and @Hazoriz, it is sinning/stealing from the husband, and therefore, ומעלה בו מעל.
"On the plane of derush, the commentary attributed to Rosh to Number (5:12) interprets it as saying that she sins against God (who is called 'ish')."
This reminded me of the case in Maharik 167:1, where a woman cheated on her husband on purpose, but thought that perhaps cheating was allowed according to Judaism, what is her status in terms of being judged as a Shogeg to stay with her husband?
He answers that since it says in the Torah that this is sinning to the husband, not to Hashem (as we noted above), since she intended to cheat on her husband, she is considered a purposeful cheater, even though she didn't know it was prohibited by Torah law.
לעניות דעתי נר' דאין לזו דין שוגגת להתירה לבעלה כיון שהיא מתכוונת למעול מעל באישה ומזנה תחתיו דהא לא כתיב איש איש כי תשטה אשתו ומעלה מעל בה' דלשתמע דוקא במכוונת לאיסר אלא ומעלה בו מעל כתיב.
Rav Hirsch in his commentary on the Chumash (ad loc and Vayikra 5:15,21) explains (I am paraphrasing) that this term was used because it relates to the robe of the Kohanim, the meil. The wife was entrusted to act faithfully, just like the Kohen who wears the meil and works in the temple, but with her actions she has shown that she is only 'wearing the clothes' ie putting on the external trappings of being a faithful wife.