Here are some admittedly idiosyncratic definitions of commonly used Buddhist terms you may run into on this site, or elsewhere in this mixed-up universe:

3 jewels/3 refuges:
Traditionally, Buddhist practitioners take refuge in the Buddha, Dhamma, and Sangha, as a way of establishing themselves on a stable & nourishing foundation.  Taking the three refuges means accepting that the changing landscape of worldly life (see 8 worldly winds, below) cannot reasonably be expected to fulfill our true hearts’ desires.  The refuges/jewels are:

Buddha – the fully awakened one.  On one level: Siddhartha Gautama, a prince in Northern India who more than 2500 years ago gave up his privileged life, went seeking the truth, found it, and then spent fifty years teaching it to anyone who would listen.  On another level: Buddha-Nature is what is already fully awakened in all beings.  Also: the Good.

Dharma/Dhamma – the teachings of the Buddha.  The way things really are.  The universal law of cause and effect.  The nature of cyclic existence as stressful, impermanent, and devoid of any fixed or separate self.  Also: the True.

Sangha – the community of those practicing towards awakening, and keeping Dhamma study & teaching alive throughout the ages.  The community of all beings, interdependent on one another.  Also: the Beautiful.

5 directions meditation:
The five directions meditation focuses your attention on the back, sides, front & center of the body, eventually bringing the whole body and heart together.  As you go through the meditation, you’ll invoke specific colors, seasons of the year, times of day, and stages of life.  You’ll also bring forth beneficial states of being, and consider the painful states they help transform.  Each time you practice the meditation, you’ll go all the way around the 5 directions, reminding yourself of their particular qualities. This is a kind of internal pilgrimage or sacred journey.  It doesn’t matter what time of day or season you find yourself in when you are doing the practice: the point is to make the whole journey each time, in order to cultivate a sense of whole, unfragmented awareness.

8 worldly winds:
These are states that arise inevitably in the course of our dealings with the world.  A wise heart understands that they are not to be taken personally or clung to, no matter how flattering or devastating they may seem.  For example, praise is inevitably linked with blame: you can’t hang on to one while pushing away the other.  The four pairs of worldly winds are:

praise & blame
fame & disrepute
pleasure & pain
gain & loss

108 names:
The number 108 is 1x2x2x3x3x3, so it has a sense of exponential growth.  Possibly because of this, it has come to signify “a lot” or “a complete set.”  The “names” part refers to the practice of attempting to describe What Is as a list of qualities or aspects of being.  For example, in Islam, there are the 99 Beautiful Names of God. If you’re curious, try googling “108 names of,” and then following the links you’ll find to the Names of Tara, the Names of Ganesh, the Names of Kali, and many other litanies (lists of holy names).  We named our site 108 Names of Now because the process of waking up is about paying attention to exactly right now, naming what is there, and resting in the gift of present awareness.

bhikkhu: a monk in the Theravada tradition. when you're reading Theravada suttas, in your mind, you can read "student" for "bhikkhu," if that helps. you can also read "she" for "he." the important thing is to feel included and addressed. this is for you! test it out in your practice, and see if it is helpful.

A bodhisattva (literally, “enlightenment being”) is committed to awakening all sentient beings to their enlightened nature, and to easing the suffering of beings everywhere.  You probably know people like this. The teacher who keeps working with the kid who struggles the most, never giving up on the kid’s ability to understand – that’s a bodhisattva.  The friend who’s seen you at your most depressed, but never lets you forget your potential?  A bodhisattva.  You, feeling the suffering that causes someone to take a swipe at you, but not swiping back?  Bodhisattva time.

Dhamma: see above, under 3 jewels

going forth into homelessness: kind of sounds bleak, but within the context of Buddhist practice, points to the possibility of being at home wherever one is, rather than only in certain controlled environments. another way of saying something similar might be: "wherever I am, my heart is there." in Buddhist texts, also refers to entering monastic life & training.

A mandala is a geometric diagram that represents a particular cosmic order, with all its parts united into a harmonious whole.  Some mandalas are viewed like looking down from overhead onto the roof of a palace, with its surrounding gardens, gates, guardians, moat, and world.  Hindu and Buddhist believers see Mt. Kailash in Tibet – with its four sides and vertical axis – as a sacred mandala manifested in the world.

A mantra is a phrase repeated as a prayer calling on a particular deity or state of mind.  For example, “om mani padme hum” (“hail to the jewel in the lotus”) is the mantra of Avalokiteshvara, the Tibetan embodiment of compassion. 

This word gets used so many ways (self-hypnosis, "tranquil escape," stress management) that it can come to feel pretty vague. The practice of Buddhist meditation:

cultivates the ability to be receptive & aware in the present moment
develops certain skillful states of mind, such as equanimity, compassion, generosity, and loving-kindness
cultivates intentions of harmlessness and benevolence
fosters insight into suffering & the roots of suffering, allowing the meditator to let go
fosters insight into happiness & the roots of happiness, allowing the meditator to enjoy them
creates new pathways of calm in the mind & heart that allow the meditator to become less reactive

There are four basic postures for meditation:

sitting is good for long periods of focused meditation. it can be done in a chair or on a cushion on the floor.  it doesn’t really matter which sitting position you choose – just be sure that you feel alert & stable, that your back is comfortably straight, and that your knees are supported.
standing is good for feeling the breath deeply and for generating energy in the body if you are feeling a little sleepy or spaced out.  stand with your feet hip-width apart and your knees a little loose, not locked. 
walking is a good practice for bringing your meditative awareness into moving around in space.  as you walk, pay attention to your breath and the flow of your body.
lying down is a good posture for feeling the whole body, and for letting go of stress.

A mudra is a teaching gesture done with your hand(s) as a way of communicating and invoking a particular truth or state of mind.  Each of the directions in the 5 directions meditation has a mudra associated with it:

north: bhumisparsa mudra – touching the earth.  equanimity and steadfastness
east: abhaya mudra – fearless compassion
south: varada mudra – generosity and joy
west: dhyani mudra – meditation & lovingkindness
center: dhammacakka mudra – awareness that contains all of the Boundless Abidings

Tathagata: another name for the Buddha. literally, it means the Thus-Come One, or the Thus-Gone One. which sounds confusing, maybe, but points to the quality of being without clinging to self & other.