What exactly is meditation?

[Aupmanyav]

Can you please expand on this?

Chidananda - eternal delight. No delight in enlightenment, only understanding.

I've never had any instruction in meditation, and don't really think of myself meditating... But when i take a walk by the river or in the forest it calmes my mind... Is this meditation?

I will term it as 'Mini Meditation". Perhaps when you forget the river and the forest, it is meditation.
Personally, I think there has to be a question and search for its answer to be a proper meditation.

Yoga Sutras of Patanjali (Ashtang Yoga, Eight-limbed Yoga):
5. Pratyahara ("Abstraction"): Withdrawal of the sense organs from external objects.
6. Dharana ("Concentration"): Fixing the attention on a single object.
7. Dhyana ("Meditation"): Intense contemplation of the nature of the object of meditation.
8. Samadhi ("Liberation"): merging consciousness with the object of meditation.

Yoga - Wikipedia

en.wikipedia.org

 

[SalixIncendium]

Chidananda - eternal delight. No delight in enlightenment, only understanding.

I've only seen this translated to consciousness-bliss (chit - consciousness; ananda - bliss)

Where are you getting "eternal delight?"

 

[Aupmanyav]

I don't. I only get 'samata' (equanimity).

"yogah sthaḥ kuru karmāṇi, saṅgaṁ tyaktvā dhanañjaya;
siddhi-asiddhyoḥ samo bhūtvā, samatvaṁ yoga ucyate." BG 2.48

Perform your duty equipoised, abandoning all attachment, O Arjuna, irrespective of success or failure. This is the yoga of equanimity.