May a Chasan, Kallah, and their acquaintances, use the term Eirusin in reference to their engagement? [i.e. אני מאורסת/Ani Meureset; Seudat Eirusin; Mesibat Eirusin, etc]
In the original Talmudic and Halachic definition, the term Eirusin, or Meureses, refers to a Halachic stage of actual marriage, in which the wife is defined as an Eishes Ish, and Biblically is prohibited from marrying another man without receiving a Kosher Get from her husband. Another term used for this stage of marriage is called Mekudeshes. In this stage of marriage, the wife is given a ring, or other object of value, after being asked to marry him with it. After she acquiesces, she becomes married to him, although may still not live with him in the same home as husband and wife, until the second stage of Nessuin takes place. It used to be in previous times that the ring giving stage of the wedding known as Eirusin, took place separately, and prior to, the second stage called Nessuin. There was often a 12-month interval between the two stages of marriage, thereby being referred to as engagement, as indeed they are engaged to become fully married as husband and wife, to live together in the same home. This system however was changed a number of generations ago, from when it became accustomed to not have any time interval, between the stages of Eirusin and Nessuin, and rather for it to all be done at the same time under the Chuppah on the day of the wedding, as we do today. Despite the above, in modern Hebrew, the term Meureses is still often used to describe a couple who is engaged. In today’s times this term simply means that both parties have agreed to eventually get married, and not that a Halachic stage of marriage has already taken place. However, in Talmudic and Halachic terminology, this stage of engagement which precedes any actual stage of marriage is referred to as Shidduchin, or Meshudeches, and not Meureses, and thus using the term Meureses to define this stage is Halachically inaccurate. It is due to this reason that the Poskim discuss whether it remains appropriate to use this term for an engaged couple, and their festivities.
The Poskim discourage using the term Meureses, or Eirusin, for an engaged couple, or in reference to any of their festivities, whether verbally or in writing. The reason for this is because the technical Halachic meaning of this term is that the couple is Halachically married, and cannot marry another person without a Kosher Get. Thus, being that we are careful in the utmost against blurring the boundaries of one who requires a Get and being potentially left as an Aguna, and one who does not, therefore, we avoid using this term. An alternative term to be used is Meshudeches, or Misibat Shidduchim and Mesibas Tanaim, which is in truth the Halachic term for their status and festivities. The above, issue however is only applicable in the Hebrew language. However, in English and other languages, there is no issue with using the term “engaged” as this term has no set Halachic definition, in contrast to the term Meureses which does.
One is to avoid using the term Meureses, or Eirusin, for an engaged couple or in reference to any of their festivities, and rather the term Meshudeches, or Misibat Shidduchim, is to be used. The above restriction is only in the Hebrew language, and in other languages the term engaged may be used freely.
 See Rambam Ishus 1:2-3; Michaber E.H. 46:8; 55:1
 Rambam ibid
 See Rambam ibid
 See Michaber E.H. 55:1; Rambam Ishus 10; Kesubos 57a
 See Michaber E.H. 56:1; Rambam Ishus 10; Kesubos 57a
 In the times of the Rishonim: See Tosafus Pesachim 102b; Sefer Hayashar 47:7; Machzor Vitru p. 587
 Tur E.H. 62; See Beis Yosef ibid; Seder Tanaim Eirusin and Nessuin chapter 1 [Levin]
 See Admur 493:1; Rama E.H. 55:1
 Igros Kodesh 26:240; Sichas Kodesh 5741 2:512 [Printed in Shulchan Menachem 4:141] “There are those who publicize an engagement of a Chasan and Kallah by writing and publicizing that so and so and so and so have become Nisarsu. This is simply a matter of ignorance, as the term Eirusin refers to when a woman becomes an Eishes Ish regarding all matters, including to matters relevant to danger of life. This is opposed to here which is a matter of Kishrei Tanaim, a name brought in Poskim. Therefore, in my opinion one is not to say or write Eirusin until it actually takes place.”; Kol Aryeh Nessuin; Nitei Gavriel 23:16
 See Gittin 88b that the mere rumor of someone being Mekudeshes could require a Get.
 See M”B 551:16