A transcriptome is the full range of messenger RNA, or mRNA, molecules expressed by an organism. The term “transcriptome” can also be used to describe the array of mRNA transcripts produced in a particular cell or tissue type. In contrast with the genome, which is characterized by its stability, the transcriptome actively changes. In fact, an organism’s transcriptome varies depending on many factors, including stage of development and environmental conditions. [1]

Transcriptomics is the study of the transcriptome—the complete set of RNA transcripts that are produced by the genome, under specific circumstances or in a specific cell—using high-throughput methods, such as microarray analysis. Comparison of transcriptomes allows the identification of genes that are differentially expressed in distinct cell populations, or in response to different treatments. [2]

Reference 

[1] https://www.nature.com/scitable/definition/transcriptome-296/

[2] https://www.nature.com/subjects/transcriptomics

[3] https://www.nature.com/articles/s41386-020-0763-3