Kotlin
JetBrains' more modern alternative to Java on the JVM (2011, v1.0 2016). Google adopted it as the primary language for Android in 2019.
Null safety in the type system (String?), data classes, extension functions, smart casts, coroutines (a lightweight alternative to threads). Fully interoperable with Java — can be mixed in the same project. Kotlin Multiplatform (KMP) shares logic between iOS/Android/web.
Dominates Android development, growing on the server (Spring supports Kotlin first-class), Gradle build scripts (gradle.kts). Competitors on the JVM: Scala (more powerful types, but complex), Groovy (legacy).