The most commonly misunderstood term is the word cousin. The meaning has changed over the years. In today's society, as a rule, a cousin is the son or daughter of an uncle or aunt. The word cousin also refers to a person who may be closely or distantly related through descent from a common ancestor.
In earlier times, particularly the seventeenth and early eighteenth centuries, the word cousin is used in many wills, deeds or guardianship records, may have a different meaning than in the modern sense. Cousin may refer to ANY relative who is not a brother, son, sister or daughter. A cousin may be a niece, nephew, aunt, uncle or any other close relative. The word cousin may lead to some dead end trails, if not properly investigated and documented.
Son-in-law and daughter-in-law also had different connotation in earlier centuries. Today, a son-in-law or daughter-in-law is a person married to a son or daughter. In the seventeenth and eighteenth centuries, a stepmother or stepfather may have referred to the child of a spouse by an earlier marriage as a son- or daughter-in-law.
Brother and sister may be difficult words to decipher in early documents, particularly in letters, diaries and institutional records. The words may actually refer to a blood brother or sister, but they may also refer to a stepbrother or stepsister, or a brother- or sister-in-law. Another frequent use of these terms refers to the relationship betweens persons of a church or a particular faith. It is best to research any people referred to as brother or sister for the actual relationship, if there is any doubt.
The terms aunt and uncle may be used to denote a blood relationship, or they may be used to imply affection and deep respect. People were often labeled aunt and uncle out of fondness. They may have been friends of the family or even relatives, perhaps a cousin!
Mother or father may actually mean mother or father in the legal sense. In earlier times, mother or father was often used to refer to the people today known as mother-in-law or father-in-law. Stepparents are often referred to as mother or father in legal documents, so it is best to prove the relationship before trying to go back another generation.
If Mrs. or Mistress is in front of a name, it is often assumed that the woman was married or widowed. This is not always the case. The term is often used as one of respect for a person's position or station in life. Instances have been found in legal documents of young women, sixteen to twenty, called Mrs. out of respect, not because of their marital status. When you are researching and find the word Mrs. on a marriage certificate, it may or may not imply widowhood.
Interestingly enough, by the early 1800's, marriage certificates did not always have the honorific in front of the bride's name. I found this out the hard way. My great-great-great grandmother Henrietta Gilly was a widow when she married my great-great-great grandfather, Jesse Fowler. This was only revealed when I found her widow's pension application for his service in the 1836 Creek Indian War. On the application, she stated that she had been a widow at the time of her marriage. I had spent over a decade researching the Gilly line, only to find that it was not mine.
The word Colonel will often be found in front of the name of a person who practiced law. Military service was often irrelevant to the title. A person was given the honorific Colonel by virtue of the profession.
Another term that can throw a person's research is the Jr. after a person's name. In today's society, Jr. implies the son, usually the first born, of a man with the same name. In earlier times, Jr. may refer to the son of a brother. Also, Jr. was often not used on the first born son. The term Jr. may not lead directly back to the father, so it is to your advantage to be wary and do your research.
The article "The OED and Your Family Tree" may be helpful in pointing to an avenue to research these terms. The main point to remember is that when doing your research, nothing should be taken at face value. Always verify your findings and document your sources. The more verification you provide for your information, the less you will find yourself climbing the wrong tree in your research.