Onboard computers are computers which are internalized within a Transformer, and assist them in maintaining their structures.
Fiction
Beast Wars
Onboard computers controlled a variety of functions, including Transformation. Usually, the activation code was spoken aloud by the Transformer in question, but they could activate it internally. Beast Wars (Part 1)
Onboard computers also controlled scans, diagnostics Power Surge and initiating stasis lock. The last component could be overridden, but it was at the Transformer's own risk. Code of Hero
Beast Machines
After being reformatted into technorganic bodies, the Maximals lost their onboard computers, thus forcing them to find an "emotional still point" to transform. This caused some problems, initially. Master of the House