Final answer:
The meter of the song 'Tonight' from Bernstein's 'West Side Story' is described as an Irregular meter, which means it does not consistently follow a pattern like duple, triple, or quadruple meter.
Step-by-step explanation:
The term that describes the meter of the song 'Tonight' from Bernstein's West Side Story is Irregular meter. Meter in music and poetry refers to the pattern of stressed and unstressed beats, akin to syllables in poetry. While other songs may fall into more consistent patterns such as duple, triple, or quadruple meter, some compositions feature varying meter throughout the piece, thus making their meter irregular. Leonard Bernstein's music often plays with unpredictable rhythmic patterns, which can contribute to the dramatic tension and emotional expression in his work, such as in 'Tonight.'