It's basically a reliable, horizontally scalable object store + a collection of data storage and processing engines.
So amongst the cloud providers, AWS calls a combination of S3 + Glue + Athena (for example) a "data lake", where S3 is the object storage which can store data in various formats, and Glue and Athena are used to transform/process/query the data. See a more detailed article/guide here: https://aws.amazon.com/lake-formation/
If you didn't want to put anything into the cloud and keep all your services on-premise, a local Hadoop cluster could be a data lake, for example using HDFS + Zookeeper + YARN + Hive.
[This is a huge over-smiplification because it's late and I really should be going home :)]
So amongst the cloud providers, AWS calls a combination of S3 + Glue + Athena (for example) a "data lake", where S3 is the object storage which can store data in various formats, and Glue and Athena are used to transform/process/query the data. See a more detailed article/guide here: https://aws.amazon.com/lake-formation/
If you didn't want to put anything into the cloud and keep all your services on-premise, a local Hadoop cluster could be a data lake, for example using HDFS + Zookeeper + YARN + Hive.
[This is a huge over-smiplification because it's late and I really should be going home :)]