This post lists and defines the words derived from the Latin term memor, which means “mindful of.”
Memory means “the power or process of retaining things experienced or learned” or refers to something, or all the things, so retained. It also pertains to information stored in a device or the capacity for doing so. Something that is significant enough in some way that it can easily be recalled is memorable (the adverbial form is memorably), while the quality or state of being easily recalled or worth recalling is memorability.
Memorial is, like memorable, an adjective pertaining to the act of recall, but it most often refers to preserving the memory of an event or person; the word is also a noun referring to an object (often a sculpture or statue) or a place that serves to do so, or a ceremony or a speech honoring a memory. (In the United States, the Memorial Day holiday honors the memory of U.S. soldiers—originally, Union soldiers during the Civil War—who have died in battle.)
Someone or something so honored is memorialized, and the action of doing so is memorialization. (A memorialist is someone who signs or writes a memorial.) The Latin phrase “in memoriam” (literally, “in memory of”) is used in English, usually in documents such as programs distributed at a memorial for a deceased person, or on gravestones honoring the dead, though the English translation is often employed.
Something that helps people remember a person or thing to be honored can also be described as being commemorative; the verb form is commemorate and the action is a commemoration.
Memorabilia, in Latin, denoted notable achievements but in English came to merely describe things worth recalling before its meaning shifted to “things that aid in recalling.” The word usually applies to souvenirs pertaining to popular culture, such as objects commemorating a form of entertainment or an athletic competition. There is no singular form, so a single object might be referred to as “a piece of memorabilia.”
Similarly, a memento is something that serves to remind one of someone or something; the plural is mementos (though mementoes is also common).
Memoir usually applies to an autobiography or biography, though it is (rarely) used to refer to a memorandum or a report. A person who writes a biographical memoir is a memoirist or a memorialist.
To memorize is to store information in one’s memory; the act is called memorization.
To remember is to place or retain information in one’s memory. Remembrance can apply neutrally to any memory, though it usually has the connotation of honoring someone or something. To disremember, meanwhile, is to forget, while to misremember is to have a faulty memory of something.
A memorandum (the word is derived directly from the Latin term meaning “to be remembered”) is a communication, often in written form, that serves as a record or reminder or that advises, directs, or informs; the plural in Latin is memoranda, though in English memorandums is prevalent, and the word is often truncated to memo (plural: memos).
The newest word in this family is meme, coined in the mid-1970s on the model of gene and defined by its coiner, scientist Richard Dawkins, as “a unit of cultural transmission,” often an idea but sometimes a behavior or style. The definition has since extended among laypeople to encompass amusing or interesting images or videos, often accompanied by audio or by written messages, disseminated through social media. The adjectival form is memetic.
Member and other words pertaining to a part or a unit are unrelated.