Yoga is all about calming down the mind. Manah Prashamanah Upaya Yoga (Yoga Vashishtha). Thought is the functional unit of Mind, so reducing the number of thoughts is calming down the mind.
Calming down the mind has cascading beneficial effects at our Physical, Mental, Emotional, Intellectual & Spiritual levels of existense (the Pancha Koshas).
Technology for calming down the mind using the body is Asana, using the breath is Pranayama & using the mind is Meditation.
The stages in calming down the mind are four:
1. first phase is when in our mind, there are multiple topics and each topic is having multiple thoughts - the monkey mind (Chanchalta)
2. When the above situation reduces to multiple thoughts related to only one topic, the stage is called Concentration (Ekagrata).
3. When our mind is engaged with just one thought with effort it is Dharana (there is no english word for this state mind)
4. When we are able to achieve the above state without effort it is called Dhyan (there is no english word for this state of mind).
While the above four stages can be thought of as gradual & serially progressive stages, there may come a point in Dhyan were there is quantum jump in consciousness and the observer, the observed and the process of observation becomes one - this is the stage of Samadhi. Samadhi is a completely different ball game and details about I will elucidate some other day. Will just like to say that there are 12 stages of Samadhi and unimaginable mental powers are acquired by the practioner in these stages of consciousness.
So, as you can see the english word meditation can be thought of relating to all five words explained above, so it is better to use the more scientific and accurate sanskrti words for the levels of consciousness - Chanchalta, Ekagrata, Dharna, Dhyan & Samadhi.
And it must be clear to you by now that posture has nothing to do with these practices and whatever posture is comfortable for the practioner, can be used for meditation. Actually any asana is good enough for meditation (including Savasana - asked by you) - different different yogis have used different different Asanas for their meditation.