Lojban is designed to be unambiguous in orthography, phonology, morphology, and grammar. Lojban semantics, however, must support the same breadth of human thought as natural languages. Every human being has different 'meanings' attached to the words they use, based on their unique personal experiences with the concepts involved. So it is impossible to eliminate semantic ambiguity (the ambiguity embedded in the variable meanings of words when taken in context) completely.
Rather, Lojban attempts to minimize semantic ambiguity, partly by systematizing as much as possible about semantics, but mostly by removing the clutter and confusion caused by other forms of ambiguity.
Unlike words in most other languages, a brivla has a single meaning, which however may encompass a narrow or broad range of closely related submeanings. gismu tend to have more general meanings, while lujvo tend to have specific definitions; the compounding of gismu into lujvo allows expression of any desired degree of specificity. fu'ivla have a single narrow meaning.
The semantic definitions of brivla are closely tied to the 'predicate' nature of brivla, a topic discussed in detail in the grammar section below. In short, a brivla defines the relationship between a group of separate but related concepts, called its sumti.
brivla are not nouns, verbs, adjectives, or adverbs; yet they incorporate elements of each. These different aspects are brought out in the way the brivla is used in the grammar, but the different grammatical environments do not change the meaning of the brivla.
brivla are an open-ended set of words; new lujvo and fu'ivla may be created as needed. Eventually, invented brivla will be collected and analyzed, and added to a formal dictionary. The definitions of all gismu, including their place structures, have already been specified. The place structure of a lujvo can generally be inferred from the place structures of its component gismu, using conventions which are generally useful though not hard-and-fast. fu'ivla are generally concrete terms, with simple and fairly obvious place structures. When there is uncertainty, listeners can ask about unknown or confusing place structures.
The heart of Lojban semantics is embedded in tanru. The meaning of a tanru is somewhat ambiguous: for instance, skami pilno could refer to a computer that is a user, or to a user of computers. There are a variety of ways that the modifier component can be related to the modified component. cmavo are used within tanru to prevent grammatical ambiguities, such as the various possible groupings of words in a phrase like pretty little girls school ({pretty {little {girls school}}}, {{pretty little} {girls school}}, {{pretty {little girls}} school}, and so on).
A speaker may use tanru to be arbitrarily general or specific. tanru are usually meant to be quite straightforward; tanru are always considered as a series of pairs of terms, a binary metaphor relationship. In such a relationship the first term by default modifies the second term. The terms may be brivla, certain cmavo such as numbers, or shorter tanru.
The connotative semantics of Lojban sentences — that is to say, the meaning contained not in the words themselves, but in the associations people make with them — is still relatively undefined. The same is true for the semantics of longer expressions or texts. There is as yet nothing clearly corresponding in Lojban to 'mood' or 'tone', no 'formal' or 'informal' styles, etc.
Because the language is oriented towards logic, the nature of the assertion in a statement, and whether it is true or false, are especially significant. Certain constructs in the language are described as making assertions, and having truth values (that is, being true or being false). Other constructs may modify those truth values, and still other constructs are interpreted independently from the truth of the statement.