Fig. 1. Modern Japanese version of the Semantic Differential. The Kanji characters in background stand for "God" and "Wind" respectively, with the compound reading "Kamikaze". (Adapted from Dimensions of Meaning. Visual Statistics Illustrated at VisualStatistics.net.)
Semantic Differential (SD) is a type of a rating scale designed to measure the connotative meaning of objects, events, and concepts. The connotations are used to derive the attitude towards the given object, event or concept.
Osgood's Semantic Differential was an application of his more general attempt to measure the semantics or meaning of words, particularly adjectives, and their referent concepts. The respondent is asked to choose where his or her position lies, on a scale between two polar adjectives (for example: "Adequate-Inadequate", "Good-Evil" or "Valuable-Worthless"). Semantic differentials can be used to measure opinions, attitudes and values on a psychometrically controlled scale.
Theoretical underpinnings of Charles E. Osgood's semantic differential have roots in the medieval controversy between the nominalists and realists. Nominalists asserted that only real things are entities and that abstractions from these entities, called universals, are mere words. The realists held that universals have an independent objective existence. Osgood's theoretical work also bears affinity to linguistics and general semantics and relates to Korzybski's structural differential.
The development of this instrument provides an interesting insight into the broader area between linguistics and psychology. People have been describing each other since they developed the ability to speak. Most adjectives can also be used as personality descriptors. The occurrence of thousands of adjectives in English is an attestation of the subtleties in descriptions of persons and their behavior available to speakers of English. Roget's Thesaurus is an early attempt to classify most adjectives into categories and was used within this context to reduce the number of adjectives to manageable subsets, suitable for factor analysis.
Osgood and his colleagues performed a factor analysis of large collections of semantic differential scales and found three recurring attitudes that people use to evaluate words and phrases: evaluation, potency, and activity. Evaluation loads highest on the adjective pair 'good-bad'. The 'strong-weak' adjective pair defines the potency factor. Adjective pair 'active-passive' defines the activity factor. These three dimensions of affective meaning were found to be cross-cultural universals in a study of dozens of cultures.
This factorial structure makes intuitive sense. When our ancestors encountered a person, the initial perception had to be whether that person represents a danger. Is the person good or bad? Next, is the person strong or weak? Our reactions to a person markedly differ if perceived as good and strong, good and weak, bad and weak, or bad and strong. Subsequently, we might extend our initial classification to include cases of persons who actively threaten us or represent only a potential danger, and so on. The evaluation, potency and activity factors thus encompass a detailed descriptive system of personality. Osgood's semantic differential measures these three factors. It contains sets of adjective pairs such as warm-cold, bright-dark, beautiful-ugly, sweet-bitter, fair-unfair, brave-cowardly, meaningful-meaningless.
Studies using the SD found additional universal dimensions. More specifically several researchers reported a factor of "Typicality" (that included scales such as "regular-rare", "typical-exclusive")  or "Reality" ("imaginary-real", "evident-fantastic", "abstract-concrete"), as well as factors of "Complexity" ("complex-simple", "unlimited-limited", "mysterious-usual"), "Improvement" or "Organization" ("regular-spasmodic", "constant-changeable", "organized-disorganized", "precise-indefinite"), Stimulation ("interesting-boring", "trivial-new").
The semantic differential is today one of the most widely used scales used in the measurement of attitudes. One of the reasons is the versatility of the items. The bipolar adjective pairs can be used for a wide variety of subjects, and as such the scale is nicknamed "the ever ready battery" of the attitude researcher. A specific form of the SD, Projective Semantics method  uses only most common and neutral nouns that correspond to the 7 groups (factors) of adjective-scales most consistently found in cross-cultural studies (Evaluation, Potency, Activity as found by Osgood, and Reality, Organization, Complexity, Limitation as found in other studies). In this method, seven groups of bipolar adjective scales corresponded to seven types of nouns so the method was thought to have the object-scale symmetry (OSS) between the scales and nouns for evaluation using these scales. For example, the nouns corresponding to the listed 7 factors would be: Beauty, Power, Motion, Life, Work, Chaos. Beauty was expected to be assessed unequivocally as "very good" on adjectives of Evaluation-related scales, Life as "very real" on Reality-related scales, etc. However, deviations in this symmetric and very basic matrix might show underlying biases of two types: scales-related bias and objects-related bias. This OSS design had meant to increase the sensitivity of the SD method to any semantic biases in responses of people within the same culture and educational background.
One problem, with this scale, is that its psychometric properties and level of measurement are disputed. The most general approach is to treat it as an ordinal scale, but it can be argued that the neutral response (i.e. the middle alternative on the scale) serves as an arbitrary zero point, and that the intervals between the scale values can be treated as equal, making it an interval scale.
A detailed presentation on the development of the semantic differential is provided in the monumental book, Cross-Cultural Universals of Affective Meaning.David R. Heise's Surveying Cultures provides a contemporary update with special attention to measurement issues when using computerized graphic rating scales. Verhagen and colleagues introduce a framework for adequate development and usage of the semantic differential. The framework advocates particular attention for collecting the set of relevant bipolar scales, linguistic testing of semantic bipolarity, and establishing semantic differential dimensionality.