Definition:

A subject that consists of two or more simple subjects joined by a coordinating conjunction (such as and or or) and that have the same predicate.

The parts of a compound subject may also be joined by correlative conjunctions, such as both . . . and and not only . . . but also.

Although both parts of a compound subject share the same verb, that verb is not always plural. See Examples and Observations below.

Also see:

• Practice in Identifying Compound Subjects
• Compound Noun
• Compound Words
• Subject-Verb Agreement
• Tricky Cases of Subject-Verb Agreement

 

Examples and Observations:

• Felix and Angie own a new Honda Accord, but they prefer to drive their old van.
• Wilbur and Orville Wright ran a printing business from their childhood home, and as young men they operated a bicycle shop.
   
• "My uncle and my cousin are both lawyers, as my father was."
  (David R. Slavitt, "Conflations." Short Stories Are Not Real Life. LSU Press, 1991)
• "A life of leisure and a life of laziness are two things. There will be sleeping enough in the grave."
  (Benjamin Franklin)
   
• Waking, running, and cycling are all aerobic activities.
• Bacon and eggs are both high in cholesterol.
• Bacon and eggs is my favorite breakfast.
• Both the director and her assistant have replied to our invitation.
• Neither the director nor her assistant has replied to our invitation.
   
• Either Paul or his cousins are coming to pick us up.
• Heads
  "We can . . . talk about the head and complete forms of compound subjects and predicates:

  My sister Laurie and her friend Katie are traveling through Europe.
  
  Playing the lottery and burning your money are equivalent activities.

  These two sentences contain compound subjects. In the first sentence, the complete compound subject is my sister Laurie and her friend Katie. The head compound subject is Laurie and Katie. Since the subject is compound, we include the conjunction in the head version. In the second sentence, the complete compound subject is playing the lottery and burning your money. The head compound subject is playing and burning."
  (Michael Strumpf and Auriel Douglas, The Grammar Bible. Henry Holt, 2004)

• Agreement With Compound Subjects
  "Normally a subject made up of more than one element takes a plural verb ("The President and Congress are at loggerheads"), although occasionally, when the elements add up to the same idea, the verb is singular ("The wear and tear on the car was tremendous"). But focus an eye on these compound subjects followed by singular verbs, all of which are correct:
   
  • Everything in the cupboard and everything on the table was smashed.
  • Everybody favoring the plan and everybody leaning toward it was interviewed.
  • Nobody in my house and nobody on my street has been robbed.
  • Anyone who has read the book and anybody who has even heard of its ideas agrees with the author.
  Strange, eh? . . . The explanation would seem to be that in each instance the second 'particularizer' is superfluous and has no grammatical effect; it could just as well be omitted, and in some of the instances the and would change to or. . . .
  
  "An odd quirk that proves nothing aside from the fact that some rules do have exceptions."
  (Theodore Bernstein, Miss Thistlebottom's Hobgoblins, 1971)
• Compound Subjects Joined by Or or Nor
  "Unlike subjects joined by 'and,' the very role of 'or' and 'nor' is to separate, to tell us that it's not both things, but one thing or the other that the verb applies to.  So the rule is:

  Subjects joined by or or nor are not considered as a group, and the verb's person and number should agree with those of the subject's individual parts.

  There are three possible scenarios here. If both parts are singular, as in the subject Mary or Donna, then the verb is singular. If they're both plural, as in the subject Neither the girls nor the boys, the verb is plural. And in really tricky sentences where you have one of each, such as Either Tony or his daughters, the verb should agree with whatever part of the subject it's closest to in the sentence; for example, either Tony or his daughters are or either the daughters or their father is. 
  (Ann Batko, When Bad Grammar Happens to Good People. Career Press, 2004)