friendship is a relationship between friends, and also the feeling or relationship that friends have, or the state of being friends; Dictionary.com adds friendly feeling or disposition.
The Oxford Advanced Learner's Dictionary marks the first sense countable, "a relationship between friends," with the example "They formed a close friendship at college." Its uncountable sense is "the feeling or relationship that friends have; the state of being friends," as in "Your friendship is very important to me." Oxford gives the origin as "Old English frēondscipe (see friend, -ship)."
Cambridge Dictionary, marking the noun countable or uncountable, defines it as "a situation in which two people are friends," with the example "From that meeting, they forged a lifelong friendship." Its American Dictionary gives "a friendly relationship, or the state of being friends."
Dictionary.com gives three senses: "the state of being a friend; association as friends," with the example "to value a person's friendship"; "a friendly relation or intimacy," with rapport, understanding, accord and harmony as synonyms; and "friendly feeling or disposition." It records the word as "First recorded before 900; Middle English; Old English frēondscipe, equivalent to friend + -ship."